Define the return type

def add(x: int, y: int) -> int:
    return x + y

first = 19
second = 23

result: str = add(first, second)

$ mypy variable_and_function_with_return_type.py
variable_and_function_with_return_type.py:8:15: error: Incompatible types in assignment (expression has type "int", variable has type "str")  [assignment]
Found 1 error in 1 file (checked 1 source file)