atcoder/abc003A

Author: xirc

atcoder/abc003/A/main.cpp

@@ -0,0 +1,21 @@
+#include <bits/stdc++.h>
+
+using namespace std;
+using ll = int64_t;
+using ff = long double;
+
+int main() {
+    ios_base::sync_with_stdio(false);
+    cin.tie(0); cout.tie(0);
+
+    int X;
+    cin >> X;
+
+    int S = 0;
+    for (int i = 1; i <= X; ++i) {
+        S += i;
+    }
+    cout << fixed << setprecision(7) << (ll(S) * 10000 / ff(X)) << endl;
+
+    return 0;
+}