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 Fortran (GNU Fortran 9.2.1) (judge verdict: Wrong Answer)

program sum_of_product
    implicit none
    integer n, i, j, sum,b, ans
    integer,allocatable :: a(:)

    read *, n
    allocate(a(n))
    read *, a
    sum=0

    do i=1,n
        do j=i+1,n
            if(j<=n)then
                if(i/=j)then
                b=a(i)*a(j)
                sum=sum+b
                end if
            else
                exit
            end if
        end do
    end do
    ans=mod(sum,1000000007)
    print *,ans
end program sum_of_product