#include <iostream>
#include <fstream>

using namespace std;

ifstream fin ("fact.in");
ofstream fout ("fact.out");

long long P;

long long nr_zero (long long n) {
    long long nr = 0;
    while (n >= 5) {
        nr += n / 5;
        n /= 5;
    }
    return nr;
}

int main() {
    fin >> P;
    long long st = 0, dr = P * 5, poz = -1;
    while (st <= dr) {
        long long m = (st + dr) / 2, zerouri = nr_zero(m);
        if (zerouri >= P) {
            dr = m - 1;
            poz = m;
        }
        else
            st = m + 1;
    }
    fout << poz << "\n";
    return 0;
}