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: Accepted)

local n=io.read("n")
local mod=1000000007
local a={}
local b={[0]=0}
for i=1,n do
    a[i]=io.read("n")%mod
    b[i]=(b[i-1]+a[i])%mod
end

local total=0
for i=1,n-1 do
    total=total+a[i]*(b[n]-b[i])%mod
end
print(total%mod)