#include <iostream>
using namespace std;
int main() {
int n;
cin >> n;
int res = n;
while (n >= 3) {
res += n / 3; // res += [当前可兑换的瓶数]
n = n / 3 + n % 3; // n = [当前可兑换的瓶数] + [兑换后剩余的瓶数]
}
cout << res << endl;
// 数学公式解答:cout << n + (n - 1) / 2 << endl;
return 0;
}