#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  if (n % 3 == 0) cout << n / 3 * 2;
  if (n % 3 == 1) cout << (n - 1) / 3 * 2 + 1;
  if (n % 3 == 2) cout << (n - 2) / 3 * 2 + 1;
}
