#include <fstream>

using namespace std;
unsigned long long x,a,b,c,p;
unsigned long long factorial(long long n)
{
  unsigned long long zero;
  zero=0;
  while(n)
  {
    zero+=n/5;
    n/=5;
  }
  return zero;
}
int main()
{
    ifstream fi("fact.in");
    ofstream fo("fact.out");
    fi>>p;
    a=1;
    b=18446744073709551614LL;
    while(a<b)
    {
      c=(a+b)/2;
      x=factorial(c);
      if(x<p) a=c+1;
      if(x>p) b=c-1;
      if(x==p) { fo<<(c/5)*5<<"\n"; break; }
    }
    fo.close();
    return 0;
}