[mypy] fix type annotations for project euler problem009/sol1

Author: pikulet

project_euler/problem_009/sol1.py

@@ -38,6 +38,8 @@ def solution() -> int:
                         if (a + b + c) == 1000:
                             return a * b * c
 
+    return -1
+
 
 def solution_fast() -> int:
     """
@@ -58,6 +60,8 @@ def solution_fast() -> int:
             if a < b < c and (a ** 2) + (b ** 2) == (c ** 2):
                 return a * b * c
 
+    return -1
+
 
 def benchmark() -> None:
     """