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

/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