#include <bits/stdc++.h>
using namespace std;
map<long long, long long> mp;
string s;
long long n, m, k;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  srand(time(0));
  cin >> n;
  cout << n / 3 * 2 + (n % 3 != 0);
}
