#include <bits/stdc++.h>
using namespace std;
const int MAXN = 100;
int main(int argc, char* argv[]) {
  ios::sync_with_stdio(0);
  int n;
  cin >> n;
  int ans = 0;
  ans /= n / 3;
  n -= ans * 3;
  for (;;) {
    if (n >= 1) {
      n -= 1;
      ans++;
    } else {
      break;
    }
    if (n >= 2) {
      n -= 2;
      ans++;
    } else {
      break;
    }
  }
  cout << ans << endl;
  return 0;
}
