#include <iostream.h>
#include <fstream.h>

long p5(int nr)
{
 long long a = 0;
 long long b = 1;
 for (a = 0;a < 27;a = a + 1)
  {
   b = b * 5;
   if ((nr % b) != 0)
     {
      return (long)(a);
     }
  }
 return 0;
}

int main(void)
{
 fstream f1("fact.in",ios::in);
 fstream f2("fact.out",ios::out);
 long long P = 0;
 long long n = 0;
 long long a = 0;
 f1 >> P;
 if (P == 0)
   {
    n = 1;
   }
  else
   {
    a = 0;
    while (n < P)
     {
      a = a + 5;
      n = n + p5(a);
     }
   }
 if (n == (n + p5(a + 5)))
   {
    f2 << -1;
   }
  else
   {
    f2 << a;
   } 
 f1.close();
 f2.close();
 return 0;
}