3047: Dreamoon and Sums
题目描述
Dreamoon loves summing up something for no reason. One day he obtains two integers a and b occasionally. He wants to calculate the sum of all nice integers. Positive integer x is called nice if 
and 
, where k is some integer number in range [1,a].
By 
we denote the quotient of integer division of x and y. By 
we denote the remainder of integer division of x and y. You can read more about these operations here: http://goo.gl/AcsXhT.
The answer may be large, so please print its remainder modulo 1000000007 (109+7). Can you compute it faster than Dreamoon?
The single line of the input contains two integers a, b (1≤a,b≤107).
Print a single integer representing the answer modulo 1000000007 (109+7).
1 1
0
2 2
8
For the first sample, there are no nice integers because 
is always zero.
For the second sample, the set of nice integers is {3,5}.