#include <fstream>

using namespace std;

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

long long p, n, st, dr, m, cm, prod, nr;

int main()
{
    fin>>p;
    if( p < 0) fout<<-1;
    else if( p == 0) fout<<1;
    else{
        st=1;
        dr=5*p;
        while( st <= dr)
        {
            m = st + (dr - st)/2;
            cm=m;
            nr=0;
            while(cm)
            {
                nr+=(cm/5);
                cm/=5;
            }
            if( p > nr) st = m + 1;
            else dr= m - 1;
        }
        fout<<st;
    }
    return 0;
}