#include <bits/stdc++.h>
using namespace std;
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long long n;
  cin >> n;
  long long cnt = 0, num = 0;
  while (n > 0) {
    n -= 1;
    cnt++;
    if (n >= 2) {
      cnt++;
      n -= 2;
    } else {
      break;
    }
  }
  cout << cnt << "\n";
  return 0;
}
