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 $1 \times 2 + 1 \times 3 + 2 \times 3 = 11$.
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