【SGU】 105. Div 3

/**
 *  @author      johnsondu
 *  @createTime  2015-8-16 17:04
    @problem     SGU Div 3
    @url         http://acm.sgu.ru/problem.php?contest=0&problem=105
    @description math, print first 20 elements and find the rule behind it.

                 0 1 2 2 3 4 4 5 6 6 ...
 */

#include <iostream>
#include <cstdio>
#include <cmath>
#include <algorithm>
#include <cstring>
#include <stack>
#include <queue>
#include <map>
#include <vector>

using namespace std;

int main()
{
    int n;
    while(cin >> n) {
        if (n < 4) cout << n - 1 << endl;
        else {
            if (n % 3 == 0 || n % 3 == 1)
                cout << n / 3 * 2 << endl;
            else 
                cout << n / 3 * 2 + 1 << endl;
        }
    }

    return 0;
}