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

ifstream f("fact.in");
ofstream g("fact.out");

long long teorema_lui_Legendre(long long x)
{
    long long sum = 0,put=1;
    while(x>=put)
    {
        put*=5;
        sum+=x/put;
    }
    return sum;
}

long long cautbin(long long p)
{
    long long m,n0,in=1,sf=100000000000;
    while (in <= sf)
    {
        m = (in+sf) / 2;
        n0 = teorema_lui_Legendre(m);
        if(n0<p)
            in=m+1;
        else if(n0>p)
            sf=m-1;
        else
        {
            while(teorema_lui_Legendre(m)==p)
                m--;
            m++;
            return m;
        }

    }
    return -1;
}

int main()
{
    long long p;
    f >> p;
    g << cautbin(p);
    return 0;
}