#include <fstream.h>

ifstream f("fact.in");
ofstream g("fact.out");
long p;

long fact (long n)
{ long nr_0=0,aux;
  for (long i=5; i<=n; i+=5)
  { aux=i; 
    while (aux%5==0) 
    { nr_0++; aux/=5; } }
  return nr_0;
}

long caut_bin (long x)
{ long hi,lo,mid;
  for (lo=1, hi=1000000; lo <= hi; )
  { mid=lo+(hi-lo)/2;
    if (x<fact(mid)) hi=mid-1;
    else if (x>fact(mid)) lo=mid+1;
    else return mid;
  }
  return -1;
}

int main ()
{ f>>p;
  long aux=caut_bin(p);
  if (aux!=-1) aux=aux-aux%5;
  g<<caut_bin(p);
  return 0;
}