Problem:
Score : 300 points

Problem Statement

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).

Constraints

2 \leq N \leq 2\times 10^5

0 \leq A_i \leq 10^9

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N
A_1 \ldots A_N

Output

Print \sum_{i=1}^{N-1}\sum_{j=i+1}^{N} A_i A_j, modulo (10^9+7).

Sample Input 1

3
1 2 3

Sample Output 1

11

We have 1 \times 2 + 1 \times 3 + 2 \times 3 = 11.

Sample Input 2

4
141421356 17320508 22360679 244949

Sample Output 2

437235829

Solution in Julia (1.4.0) (judge verdict: Accepted)

function main()
    S = parse(Int, readline())
    T = parse.(Int, split(readline()))

    ret = 0
    #println(typeof(ret))
    ruiseki = 0
    for i in S:-1:2
        #println(i)
        ruiseki += T[i]
        ruiseki = ruiseki % (10 ^ 9 + 7)
        ret += (T[i - 1] * ruiseki) % (10 ^ 9 + 7)
        ret = ret % (10 ^ 9 + 7)
        #println(ret)
    end
    print(ret % (10 ^ 9 + 7))
end

main()