#include <iostream>
#include <fstream>
using namespace std;
ifstream fin ("fact.in");
ofstream fout ("fact.out");
int p,sol=-1;
int pow(int a)
{
    long long q=5;
    int c=0;
    while (q<=a)
          {c=c+a/q;
           q*=5;}
    return c;
}
int main()
{
    fin >>p;
    int left=1,right=5*p;
    while (left<=right)
          {int mid=(left+right)/2;
           if (p<=pow(mid)) {right=mid-1;
                             if (pow(mid)==p) sol=mid;}
           else {left=mid+1;}}
    if (p==0) sol=1;
    fout <<sol;
    return 0;
}