#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  int ans = 0;
  if (n >= 3) {
    ans = ans + (n / 3) * 2;
    n = n % 3;
  }
  if (n > 0) ans++;
  cout << ans << endl;
}
