#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
int main() {
  int n;
  scanf("%d", &n);
  int q = n / 3;
  int r = n % 3;
  if (r > 0) {
    printf("%d", 2 * q + 1);
  } else {
    printf("%d", 2 * q);
  }
  return 0;
}
