#include <bits/stdc++.h>
using namespace std;
const long long MOD = 1e9 + 7;
const int N = 1e5 + 7, M = 1e7, OO = 0x3f3f3f3f;
int main() {
  int n;
  cin >> n;
  cout << (n / 3) * 2 + (bool)(n % 3);
  return 0;
}
