#include <fstream>
using namespace std;
ifstream f("fact.in");
ofstream g("fact.out");
int li,ls,mij,sol,n,p,k;
int main()
{
    f>>p;
    li=1;
    ls=500000000;
    sol=-1;
    while(li<=ls)
    {
        mij=(li+ls)/2;
        k=0;
        n=mij;
        while(n>0)
        {
            n=n/5;
            k=k+n;
        }
        if(k<p)
        li=mij+1;
        else
        if(k>p)
        ls=mij-1;
        else
        sol=mij,ls=mij-1;
    }
    g<<sol;
    return 0;
}