#include <bits/stdc++.h>
using namespace std;
const int MAX_N = 1e5 + 5;
int n;
void solve() {
  int cnt = (n / 3) * 2;
  n %= 3;
  if (n > 0) cnt++;
  cout << cnt << "\n";
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int t = 1;
  while (t--) {
    cin >> n;
    solve();
  }
  return 0;
}
