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 PyPy3 (7.3.0) (judge verdict: Accepted)

mod=1000000007
n=int(input())
#n,k=map(int,input().split())
a=[int(i) for i in input().split()]
t=[0]*(n+1)
for i in range(n):
    t[i]=t[i-1]+a[i]
ans=chk=0
#print(t)
for i in range(n-1):
#    print(-(i+1))
    x=-1*(i+1)
#    print(x)
#    print(a[x],t[x-2])
    ans+=a[x]*t[x-2]
    ans%=mod
print(ans)