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