#include <bits/stdc++.h>
#pragma comment(linker, "/STACK:256000000")
using namespace std;
const int MAX = 0;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  int n;
  cin >> n;
  if (n < 3) {
    int res = 1;
    cout << res;
    return 0;
  }
  int res = n / 3 * 2;
  int k = n % 3;
  if (k != 0) res++;
  cout << res;
  return 0;
}
