import java.util.Scanner;

public class Main {
	public static void main(String args[]) {

		Scanner sc = new Scanner(System.in);

		//標準入力取得
		int num = sc.nextInt();
		int d[] = new int[num];
		for (int i = 0; i < num; i++) {
			d[i] = sc.nextInt();
		}

		int total = 0;
		for (int i = 0; i < num - 1; i++) {
			for (int j = i + 1; j < num; j++) {
				total = total + d[i] * d[j];
			}
		}
		System.out.println(total);

		sc.close();
	}
}

/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