def main():

    N = int(eval(input()))

    if N % 2 == 1:

        return 0



    ret = 0

    for p in range(1, 30 + 1):

        ret += N // (2 * pow(5, p))

    return ret





if __name__ == '__main__':

    print((main()))

N = int(eval(input()))

if N % 2 == 1:

    print((0))

else:

    N //= 2

    print((sum(N // pow(5, j) for j in range(1, 30))))

def main():
    N = int(eval(input()))
    if N % 2 == 1:
        return 0
    ret = 0
    for p in range(1, 30 + 1):
        ret += N // (2 * pow(5, p))
    return ret


if __name__ == "__main__":
    print((main()))

N = int(eval(input()))
if N % 2 == 1:
    print((0))
else:
    N //= 2
    print((sum(N // pow(5, j) for j in range(1, 30))))

-def main(): -    N = int(eval(input())) -    if N % 2 == 1: -        return 0 -    ret = 0 -    for p in range(1, 30 + 1): -        ret += N // (2 * pow(5, p)) -    return ret - - -if __name__ == "__main__": -    print((main())) +N = int(eval(input())) +if N % 2 == 1: +    print((0)) +else: +    N //= 2 +    print((sum(N // pow(5, j) for j in range(1, 30))))