#include <iostream>
#include <fstream>

using namespace std;

int main()
{
    long p,n,t,i;
    int ok,o;
    ifstream f("fact.in");
    ofstream g("fact.out");
    f>>p;
    ok=0;
    n=1;
    while (ok==0)
    {
        t=1;
        for(i=1;i<=n;i++)
            t=t*i;
        o=0;
        while (p%10==0)
            o++;
        if(t=o)
            ok=1;
        else
            n++;
    }
    g<<n;
    return 0;
}