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(8) n, i, j, b, sum,ans,x
    integer(8),allocatable :: a(:)

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

    if(2<=n .or. n<=200000)then
        do i=1,n-1
            if(0<=a(i) .or. a(i)<=1000000000)then
                do j=1+i,n
                        x=mod(a(i),1000000007)
                        b=a(j)*x
                        sum=sum+b
                end do
            end if
        end do
    end if
    ans=mod(sum,1000000007)
    print *,ans
end program sum_of_product