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