#include <bits/stdc++.h>
using namespace std;
inline int read() {
  int ret = 0, f = 0;
  char c = getchar();
  while (!isdigit(c)) {
    if (c == '-') f = 1;
    c = getchar();
  }
  while (isdigit(c)) {
    ret = ret * 10 + c - 48;
    c = getchar();
  }
  if (f) return -ret;
  return ret;
}
int main() {
  int n = read();
  cout << (n * 2 + 1) / 3;
}
