import java.util.Scanner;

public class Main {
  public static void main (String[] args) {
    Scanner in = new Scanner(System.in);

    while (in.hasNext()) {
      int N = in.nextInt();
      int[] d = new int[N];

      for (int i = 0; i < d.length; ++i) {
        d[i] = in.nextInt();
      }

      int sum = 0;

      for (int i = 0; i < d.length; ++i) {
        for (int j = i + 1; j < d.length; ++j) {
          sum += d[i] * d[j];
        }
      }

      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