#include <iostream>
#include <fstream>
using namespace std;

int main()
{ifstream f("fact.in");
ofstream g("fact.out");
    int p,nr=0,d=0,ok=0,n=-1,fac=1,i;
    f>>p;
    while(ok==0)
    {
        d++;
        for(i=1;i<=d;i++)
            fac=fac*i;
        while(fac%10==0)
        {
            fac=fac/10;
            nr++;
        }
        if(p==nr)
        {
         ok=1;
         n=i;
        }
    }
    g<<n;
    return 0;
}