题目链接：http://acm.hdu.edu.cn/showproblem.php?pid=2018

设 f[i][j] 表示第i天年龄为j的母牛个数，其中j=4代表所有年龄达到4岁的成年母牛，则：

• f[1][4] = 1
• f[1][i] = 0, i = 1,2,3
• f[i][4] = f[i-1][4] + f[i-1][3]
• f[i][3] = f[i-1][2]
• f[i][2] = f[i-1][1]
• f[i][1] = f[i][4]

代码：

#include <iostream>

#include <string>

using namespace std;

const int maxn = 56;

long long f[maxn][5];

void init() {

    f[1][4] = 1;

    for (int i = 2; i < maxn; i ++) {

        f[i][4] = f[i-1][4] + f[i-1][3];

        f[i][3] = f[i-1][2];

        f[i][2] = f[i-1][1];

        f[i][1] = f[i][4];

    }

}

void output(int i) {

    cout << f[i][1] + f[i][2] + f[i][3] + f[i][4] << endl;

}

int n;

int main() {

    init();

    //for (int i = 1; i <= 10; i ++) output(i);

    while (cin >> n) {

        if (!n) break;

        output(n);

    }

    return 0;

}