import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

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

		int num = sc.nextInt();
		int hp = 0;
		List<Integer> takoyaki = new ArrayList<Integer>();
		takoyaki.add(sc.nextInt()); //1つ目のたこ焼き
		for (int i = 1; i < num; i++) {
			takoyaki.add(sc.nextInt());
			for (int j = 0; j < i; j++) {
				hp = hp + takoyaki.get(i) * takoyaki.get(j);
			}
		}
		System.out.println(hp);

	}
}
/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