#include <bits/stdc++.h>
using namespace std;
using uc = unsigned char;
using ui = unsigned int;
using ul = unsigned long long int;
using sc = signed char;
using si = signed int;
using sl = signed long long int;
using ld = long double;
void m_assert(const bool& cond, const sl& line, const char* condstr) {
  if (!cond) {
    cerr << "Assertion Failed: " << condstr << " at line " << line << endl;
    exit(1);
  }
}
static sl N;
sl traing_zeroes(sl n) {
  sl res = 0;
  for (sl i = 5; n / i >= 1; i *= 5) {
    res += n / i;
  }
  return res;
}
void solve(void) {
  if (N % 2 == 1) {
    cout << 0 << endl;
    return;
  }
  cout << traing_zeroes(N / 2) << endl;
}
int main(void) {
  cin >> N;
  solve();
  return 0;
}
