#include<iostream>
using namespace std;
int main()
{
  long long count=0,N,S=5;
  cin>>N;
  count=N/10;
  if(N%2==1){
  cout<<0<<endl;
    return 0;
  }
  N/=10;
  while(N/S>0){
    count+=N/S;
    S*=5;
  }
  cout<<count<<endl;
  
}
