import java.util.Scanner;

class Main {

	public static void main(String[] args) {

		Scanner scan = new Scanner(System.in);
		int n = scan.nextInt();
		int d[] = new int[n];
		for (int i = 0; i < n; i++)
			d[i] = scan.nextInt();

		scan.close();
		int s = 0;
		for (int a = 0; a < n; a++) {
			for (int b = a + 1; b < n; b++) {
				s = s + d[a] * d[b];
			}
		}
		System.out.print(s);
	}
}
/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