import sys


def next_str() -> str:
    result = ""
    while True:
        tmp = sys.stdin.read(1)
        if tmp.strip() != "":
            result += tmp
        elif tmp != '\r':
            break
    return result


def next_int() -> int:
    return int(next_str())


def main() -> None:
    hand = next_int() * 60
    run_cost = next_int()
    init_cost = next_int() * 3600

    if hand <= run_cost:
        print(-1)
        return

    print((init_cost - 1) // (hand - run_cost) + 1)


if __name__ == '__main__':
    main()