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 Lua (Lua 5.3.5) (judge verdict: Wrong Answer)

-- Atcoder https://atcoder.jp/contests/abc177/tasks/abc177_c

local n = tonumber(io.read())
local modulo = 10^9 + 7

local list = {}
for i = 1, n do
    list[i] = io.read("n")
end

local sum = 0

for i = 1, n do
    for j = i + 1, n do
        sum = (sum + list[i] * list[j])
        if sum > modulo * 100 then sum = sum - modulo * 100 end
        if sum > modulo * 10 then sum = sum - modulo * 10 end
        if sum > modulo then sum = sum - modulo end
    end
end

sum = sum % modulo
print(string.format("%d", sum))