import java.util.*;

class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    sc.nextLine();
   	String d[] = sc.nextLine().split(" ", 0);
    int sum =0;
    
    for(int i=0;i<n-1;i++){
      for(int j=i+1;j<n;j++){
        int a =Integer.parseInt(d[i]);
        int b =Integer.parseInt(d[j]);
        sum+=a*b;
      }
    }
    
    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