Score : 400 points
We have N+1 integers: 10^{100}, 10^{100}+1, ..., 10^{100}+N.
We will choose K or more of these integers. Find the number of possible values of the sum of the chosen numbers, modulo (10^9+7).
Input is given from Standard Input in the following format:
N K
Print the number of possible values of the sum, modulo (10^9+7).
3 2
10
The sum can take 10 values, as follows:
200000 200001
1
We must choose all of the integers, so the sum can take just 1 value.
141421 35623
220280457