#include <bits/stdc++.h>
int main() {
  int n;
  scanf("%d", &n);
  if (n == 2 || n == 1) {
    printf("1\n");
  } else {
    int a = n / 3 * 2;
    if (n % 3 != 0) a++;
    printf("%d\n", a);
  }
  return 0;
}
