#include <iostream>
#include <fstream>

using namespace std;

int main()
{
    ifstream f("fact.in");
    ofstream g("fact.out");
    long int p;
    f >> p;

    long int n = 0;
    long int db = 0;
    long int s = 1;
    while (db<p) {
            n++;
            s = s * n;
            while(s%10==0) {
                s = s/10;
                db++;
            }
            s = s%10;

    }
    g << n;
    return 0;
}