def find_missing(arr: list) -> int:
    init = 1
    for n in arr:
        if n == init:
            init += 1
        else:
            return init


def main():
    print(f"Тест 1: {find_missing([1, 2, 3, 5, 6, 7])}")
    print(f"Тест 2: {find_missing([2, 3, 4, 5])}")
    print(f"Тест 3: {find_missing([1, 2, 3, 4, 5, 7, 8])}")


if __name__ == '__main__':
    main()