hdu2045

借鉴：：http://hi.baidu.com/sunnyykn/item/a71f3016269089ed9813d6ea

#include<iostream>
using namespace std;
int main()
{
    __int64 a[55] = {0};
    a[1] = 3; a[2] = a[3] = 6;
    for(int i = 4; i <= 50; i++)
    {
        a[i] = a[i-1] + 2*a[i-2];
    }
    int n;
    while(cin >> n)
        cout << a[n] << endl;
}