import java.util.Scanner;

public class Main{
	public static void main(String[] args) {
		int n;
		int[] d;

		Scanner sc = new Scanner(System.in);
		n = sc.nextInt();

		d = new int[n];

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

		int sum = 0;
		int right = 0;

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

		System.out.println(sum);

		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