import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner s = new Scanner(System.in);
		int amount = s.nextInt();
		int[] nums = new int[amount];
		int total = 0;
		for(int i = 0; i < amount; i++) {
			nums[i] = s.nextInt();
		}
		for(int i = 0; i < amount; i++) {
			for(int j = 1+i; j < amount; j++) {
				total += nums[i] * nums[j];
			}
		}
		System.out.println(total);
	}
}

/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