
import java.util.*;
import java.lang.*;

class Main {
	public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int d[] = new int[N];
        int recover = 0;

        for(int i = 0; i  < N; i++){
          d[i] = sc.nextInt();
        }

        for(int i = 0; i < N-1 ; i++){
          int a = d[i];
          for(int j = i+1; j < N; j++){
            recover += a * d[j];
          }
        }

        System.out.println(recover);



    }
}





/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