#include <iostream>
#include <fstream>
using namespace std;
ifstream in("fact.in");
ofstream out("fact.out");
long long n;
long long nr0(long long p)
{
    long long cnt=1;
    long long s=0;
    while(p/cnt>0)
    {
        cnt=cnt*5;
        s=s+p/cnt;
    }
    return s;
}
int main()
{
    long long p;
    in>>p;
    long long x = 100000000000000000/2;
    long st=1,dr=x*2,mij=x;
    while(st<=dr)
    {
        mij=(st+dr)/2;
        if(nr0(mij)==p)
        {
            n=mij;
            break;
        }
        else if(nr0(mij)<p)
            st=mij+1;
        else
            dr=mij-1;
    }
    if(n!=0)
    {
        while(n>0 && nr0(n)==p)
            n--;
        out<<n+1;
    }
    else
        out<<-1;
    return 0;
}