import java.util.*;
class Main{
    public static void main(String args[]){
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int d[] = new int[N];
        int ans = 0;
        for(int i=0;i<N;i++){
            d[i]=sc.nextInt();
        }
        for(int i=0;i<N;i++){
            for(int j=0;j<i;j++){
                ans+=d[i]*d[j];
            }
        }
        System.out.print(ans);
    }
}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz