import java.util.Scanner;
public class Main {
    public static void main (String[] args) {
      
      Scanner scanner = new Scanner(System.in);
      int n = scanner.nextInt();
      String dmmy = scanner.nextLine();
      String d_all = scanner.nextLine();
      String[] d = d_all.split(" ");
      
      int r_int = 0;

      for (int i = 0;  i < n - 1 ; i ++ ) {
      for (int j = i+1;  j < n ; j ++ ) {
        r_int = r_int + (Integer.parseInt(d[i]) * Integer.parseInt(d[j]));
      }
      }
          System.out.println(r_int);

      return;
    }
  }

/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