#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, z = 0;
  cin >> n;
  while (n != 0) {
    if (n >= 2)
      n -= 2;
    else if (n == 1)
      n -= 1;
    z++;
  }
  cout << z;
  return 0;
}
