Score : 300 points
Given are N integers A_1,\ldots,A_N.
Find the sum of A_i \times A_j over all pairs (i,j) such that 1\leq i < j \leq N, modulo (10^9+7).
Input is given from Standard Input in the following format:
N A_1 \ldots A_N
Print \sum_{i=1}^{N-1}\sum_{j=i+1}^{N} A_i A_j, modulo (10^9+7).
3 1 2 3
11
We have 1 \times 2 + 1 \times 3 + 2 \times 3 = 11.
4 141421356 17320508 22360679 244949
437235829