import java.util.*;
class Main {
    public static void main(String args[]){
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] b = new int[n];
        for (int i =0;i<n;i++){
            b[i] = sc.nextInt();
        }
        System.out.println(permutation(b));
        sc.close();
    }
    public static int permutation(int[] q){
        int ret = 0;
        for (int var = 0 ; var < q.length-1 ; var++) {
            for (int var2 = 0; var2 < q.length ;var2++) {
                if(var < var2) ret += q[var] * q[var2];
              
              //System.out.println(var + " " + var2 +  " " + ret);
            }
        }
        return ret;
    }
}


/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