#include <iostream>
#include <fstream>


long long factorial(long long n){
    int contor = 0;

    while (n > 0){
        n /= 5;
        contor++;
    }

    return (contor * (contor - 1) / 2);
}

int main(void){

    std::ifstream in;
    std::ofstream out;
    long long result = 0;
    long long n;

    in.open("fact.in");
    out.open("fact.out");

    in >> n;
    result = (n - factorial(n) );
    out << result;

    in.close();
    out.close();
    return 0;
}