403 D. Beautiful Pairs of Numbers
/* 题意:给定n和k,问满足1<=a1<=b1<a2<=b2<a3<=b3<...<ak<=bk<=n,并且b1-a1,b2-a2,b3-a3...bk-ak互不相同的组数. 假设x1=b1-a1,x2=b2-a2,x3=b3-a3,...,xk=bk-ak 则可以得到a1+x1+...+xk+k-1<=n,我们要求它的非负整数解 也就是要求x1+x2+...+xk<=n-a1+1的正整
