AtCoder ABC177 C.Sum of product of pairs

[sum_{i=1}^{N-1}sum_{j=i+1}^NA_iA_j=sum_{i=1}^NA_i*(A_{i+1}+cdots+A_N) ]

const int N=2e5+10;
int a[N];
LL sum[N];
int n;

int main()
{
    cin>>n;

    for(int i=1;i<=n;i++)
    {
        cin>>a[i];
        sum[i]=sum[i-1]+a[i];
    }

    int res=0;
    for(int i=1;i<=n;i++)
    {
        int t=(sum[n]-sum[i])%mod;
        res=(res+(LL)a[i]*t)%mod;
    }

    cout<<res<<endl;
    //system("pause");
    return 0;
}