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 ans = 0;

    for(int i = 0; i < n; i++){
      for(int j = i;  j < n;j++){
        if(i == j){
          continue;
        }
        ans += d[i] * d[j];
      }
    }

    System.out.println(ans);
  }
}

/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