import java.util.Scanner;

public class Main {

	public static void main(String[] args) {

		Scanner sc = new Scanner(System.in);

		int n = sc.nextInt();
		int[] ns = new int[n];
		int s = 0;

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

		for(int i = 0; i < n - 1; i++) {
			for(int j = 1 + i; j < n; j++) {
				s += ns[i] * ns[j];
			}
		}
		System.out.println(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