import java.util.Scanner;
public class Main {
  public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
       	int[] d = new int[n];
       	for(int i = 0 ; i < n ; i++ ){
        	d[i] = sc.nextInt();
        }
    	int s = 0;
       	for(int i = 0 ; i < n ; i++ ){
            for(int j = 0 ; j < i ; j++ ){
            	s += d[i] * d[j];
            }
        }
        System.out.println(s);
  	}
}
/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