#include <iostream>
#include <fstream>
using namespace std;
ifstream f("fact.in");
ofstream g("fact.out");
long long b,c,a,m,P,i;
int main()
{ f>>P;

 for (i=1;c!=P;i++)  {m=i;
 while (m%2==0) {a++;m=m/2;}
 while (m%5==0) {b++;m=m/5;}
if (a>b) {a=a-b;c=c+b;b=0;}
if (b>a) {b=b-a;c=c+a;a=0;}
if (a==b) {c=c+a;b=0;a=0;}
}
g << i-1;
    return 0;
}