#include <bits/stdc++.h>
using namespace std;
int main() {
  long long n, c = 0;
  cin >> n;
  while (n > 0) {
    if (n - 2 >= 0) {
      n -= 2;
      c++;
    }
    if (n - 1 >= 0) {
      n--;
      c++;
    }
  }
  cout << c;
  return 0;
}
