#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int n, res = 0;
  bool turn = true;
  cin >> n;
  while (n > 0) {
    if (turn) {
      n--;
    } else {
      n -= 2;
      if (n < 0) {
        break;
      }
    }
    res++;
    turn = !turn;
  }
  cout << res << "\n";
  return 0;
}
