diff --git a/Cargo.lock b/Cargo.lock
index 4f7d2c1..1369cc5 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -100,6 +100,7 @@ dependencies = [
  "superslice 1.0.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "take_mut 0.2.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "text_io 0.1.7 (registry+https://github.com/rust-lang/crates.io-index)",
+ "union-find 0.3.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "whiteread 0.5.0 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
@@ -998,6 +999,11 @@ name = "unicode-xid"
 version = "0.2.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 
+[[package]]
+name = "union-find"
+version = "0.3.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
 [[package]]
 name = "version_check"
 version = "0.1.5"
@@ -1157,6 +1163,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 "checksum unicode-segmentation 1.6.0 (registry+https://github.com/rust-lang/crates.io-index)" = "e83e153d1053cbb5a118eeff7fd5be06ed99153f00dbcd8ae310c5fb2b22edc0"
 "checksum unicode-xid 0.1.0 (registry+https://github.com/rust-lang/crates.io-index)" = "fc72304796d0818e357ead4e000d19c9c174ab23dc11093ac919054d20a6a7fc"
 "checksum unicode-xid 0.2.0 (registry+https://github.com/rust-lang/crates.io-index)" = "826e7639553986605ec5979c7dd957c7895e93eabed50ab2ffa7f6128a75097c"
+"checksum union-find 0.3.2 (registry+https://github.com/rust-lang/crates.io-index)" = "ded8a2ba8917ff7a977a446770a8c3bfec18e147b5617b41fd8aae703d8c6720"
 "checksum version_check 0.1.5 (registry+https://github.com/rust-lang/crates.io-index)" = "914b1a6776c4c929a602fafd8bc742e06365d4bcbe48c30f9cca5824f70dc9dd"
 "checksum version_check 0.9.1 (registry+https://github.com/rust-lang/crates.io-index)" = "078775d0255232fb988e6fccf26ddc9d1ac274299aaedcedce21c6f72cc533ce"
 "checksum wasi 0.7.0 (registry+https://github.com/rust-lang/crates.io-index)" = "b89c3ce4ce14bdc6fb6beaf9ec7928ca331de5df7e5ea278375642a2f478570d"
diff --git a/Cargo.toml b/Cargo.toml
index a539a9e..5ca8954 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -133,6 +133,9 @@ im-rc = "=14.0.0"
 fixedbitset = "=0.2.0"
 bitset-fixed = "=0.1.0"
 
+# union-find (a.k.a. disjoint-set)
+union-find = "=0.3.2"
+
 # 競技プログラミングの入出力サポート
 proconio = { version = "=0.3.4", features = ["derive"] }
 text_io = "=0.1.7"
diff --git a/examples/abc120-d.rs b/examples/abc120-d.rs
new file mode 100644
index 0000000..27810d2
--- /dev/null
+++ b/examples/abc120-d.rs
@@ -0,0 +1,29 @@
+// https://atcoder.jp/contests/abc120/tasks/abc120_d
+
+use proconio::marker::Usize1;
+use proconio::{fastout, input};
+use union_find::{QuickFindUf, UnionBySize, UnionFind as _};
+
+#[fastout]
+fn main() {
+    input! {
+        n: usize,
+        m: usize,
+        abs: [(Usize1, Usize1); m],
+    }
+
+    let mut u = QuickFindUf::<UnionBySize>::new(n);
+    let mut k = n * (n - 1) / 2;
+    let mut r = vec![k];
+    r.extend(abs.into_iter().rev().map(|(a, b)| {
+        let p = u.get(a).size() * u.get(b).size();
+        if u.union(a, b) {
+            k -= p;
+        }
+        k
+    }));
+    assert_eq!(r.pop(), Some(0));
+    for r in r.into_iter().rev() {
+        println!("{}", r);
+    }
+}
diff --git a/examples/tests.ron b/examples/tests.ron
index 445367d..2b4f9ab 100644
--- a/examples/tests.ron
+++ b/examples/tests.ron
@@ -12,5 +12,9 @@
             name: "ABC049 / ARC065: C - 白昼夢 / Daydream",
             matching: ExactWords,
         ),
+        "abc120-d": (
+            name: "ABC120: D - Decayed Bridges ",
+            matching: ExactWords,
+        ),
     }
 )
diff --git a/examples/testsets/abc120-d/in/sample_01.txt b/examples/testsets/abc120-d/in/sample_01.txt
new file mode 100644
index 0000000..8881ee2
--- /dev/null
+++ b/examples/testsets/abc120-d/in/sample_01.txt
@@ -0,0 +1,6 @@
+4 5
+1 2
+3 4
+1 3
+2 3
+1 4
diff --git a/examples/testsets/abc120-d/in/sample_02.txt b/examples/testsets/abc120-d/in/sample_02.txt
new file mode 100644
index 0000000..23a7d58
--- /dev/null
+++ b/examples/testsets/abc120-d/in/sample_02.txt
@@ -0,0 +1,6 @@
+6 5
+2 3
+1 2
+5 6
+3 4
+4 5
diff --git a/examples/testsets/abc120-d/in/sample_03.txt b/examples/testsets/abc120-d/in/sample_03.txt
new file mode 100644
index 0000000..c9fa102
--- /dev/null
+++ b/examples/testsets/abc120-d/in/sample_03.txt
@@ -0,0 +1,2 @@
+2 1
+1 2
diff --git a/examples/testsets/abc120-d/out/sample_01.txt b/examples/testsets/abc120-d/out/sample_01.txt
new file mode 100644
index 0000000..ce56183
--- /dev/null
+++ b/examples/testsets/abc120-d/out/sample_01.txt
@@ -0,0 +1,5 @@
+0
+0
+4
+5
+6
diff --git a/examples/testsets/abc120-d/out/sample_02.txt b/examples/testsets/abc120-d/out/sample_02.txt
new file mode 100644
index 0000000..83dd8d4
--- /dev/null
+++ b/examples/testsets/abc120-d/out/sample_02.txt
@@ -0,0 +1,5 @@
+8
+9
+12
+14
+15
diff --git a/examples/testsets/abc120-d/out/sample_03.txt b/examples/testsets/abc120-d/out/sample_03.txt
new file mode 100644
index 0000000..d00491f
--- /dev/null
+++ b/examples/testsets/abc120-d/out/sample_03.txt
@@ -0,0 +1 @@
+1
diff --git a/tools/test-with-generated-opts/src/main.rs b/tools/test-with-generated-opts/src/main.rs
index 0928fc7..f78d031 100644
--- a/tools/test-with-generated-opts/src/main.rs
+++ b/tools/test-with-generated-opts/src/main.rs
@@ -179,8 +179,7 @@ fn test(task_name: &str, matching: Matching, testsets: &Path, binary: &Path) ->
             .with_context(|| format!("Failed to execute {}", binary.display()))?;
 
         child.stdin.as_mut().unwrap().write_all(input.as_ref())?;
-        let status = child.wait()?;
-        let stop = Instant::now();
+        child.stdin.take();
         let actual = {
             let mut actual = "".to_owned();
             child
@@ -191,6 +190,8 @@ fn test(task_name: &str, matching: Matching, testsets: &Path, binary: &Path) ->
                 .with_context(|| format!("{} outputted invalid UTF-8", binary.display()))?;
             actual
         };
+        let status = child.wait()?;
+        let stop = Instant::now();
 
         let time = (stop - start).as_millis();
         let verdict = if status.success() && matching.accepts(&expected, &actual) {