#include <fstream>
using namespace std;
#define max1 9223372036854775807
#define lung unsigned long long int
#define lung1 long long int

lung fact(lung n) {
  lung p = 0;
  
  for(int i = 1; 5*i <= n; i = i*5) {
    p += n / 5*i;
  }
  return p;
}      

lung1 binary(lung x, lung st, lung dr) {
  lung mid = fact((st + dr) / 2);
  if(st == dr) {
    if(x == mid)
      return mid;
    else
      return -1;
  }
  else {
    if(x > mid)
      return binary(x, st, mid-1);
    else
      return binary(x, mid+1, dr);
  }
}

int main(void) {
  ifstream in("fact.in");
  ofstream out("fact.out");
  lung p;
  in >> p;

  if(p >= 0) {
    lung n = binary(p, 0, max1);
    out << n;
  }
  else {
    out << -1;
  }
}