这不已然是动态转移方程了么
敬仰~~~

> 预处理： 
>         F(n,k)  代表W形的以k开始的planks数目为n的Fence 
>         G(n,k)  代表M形的以k开始的planks数目为n的Fence 
>         
>         F(n,k) = sigma(G(n-1, i)) (1 <= i <= k-1)
>         G(n,k) = sigma(F(n-1, i)) (k <= i <= n-1)
>         F(1,1) = G(1,1) = 1;
>         G(n,k) = F(n,k) = 0 (k > n).