poj2663

递推，注意f[0] = 1;

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
using namespace std;

#define maxn 31

int main()
{
//    freopen("t.txt", "r", stdin);
    int f[maxn] = {1,0,3,0};
    for (int i = 4; i < maxn; i++)
    {
        f[i] = f[i - 2] * 3;
        for (int j = 4; i - j >= 0; j += 2)
            f[i] += f[i - j] * 2;
    }
    int n;
    while (scanf("%d", &n), n != -1)
        printf("%d\n", f[n]);
    return 0;
}