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