题意
做法
令(N=lfloorfrac{n-1}{2} floor)：(ans=sumlimits_{i=1}^{N}sumlimits_{j=2 i+1}^n(y-2)!{j-2choose y-2}(n-y)!=(y-2)!(n-y)!sumlimits_{i=1}^{N}({n-1choose y-1}-{2i-1choose y-1}))
令(A_{y-1}={1choose y-1}+{3choose y-1}+...+{2N-1choose y-1}，B={2choose y-1}+{4choose y-1}+...+{2Nchoose y-1})
有(A_{y-1}+B_{y-1}={2N+1choose y})，(A_{y-1}+A_{y-2}=B_{y-1})，则：
[A_{y-1}=frac{{2lim+1choose y}-A_{y-2}}{2}
]
则(A)可以(O(n))递推出来，预处理即可