#include <iostream>
using namespace std;

int maxPowerOf(int p, int n)
{
    int k = 0;
    int powerOfP = 1;
    while (true)
    {
        powerOfP *= p;
        int addend = n / powerOfP;
        if (addend == 0)
            return k;
        k += addend;
    }
}

int main() {
int n = 0;
cin >> n;
int p5 = maxPowerOf(5, n);
cout << (p5/2) << '\n';
}