import java.util.*;
public class Main{
 public static void main(String[] args){
   Scanner sc = new Scanner(System.in);
   int N = sc.nextInt();
   int[] tako = new int[N];
   for(int i =0;i<N;i++) 
   {
     tako[i] = sc.nextInt();
   }
   int sum = 0;
   for(int i = 0;i<N;i++){
     for(int k = i+1;k<N;k++){
       sum+= tako[i]*tako[k];
     }
   }
   System.out.println(sum);
 }
}
/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