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