import java.util.Scanner;


public class Main {
	public static void main (String args[]) throws Exception{
		Scanner scan = new Scanner(System.in);
		int n = scan.nextInt();
		int d[] = new int[n];

		int ans = 0;
		
		for(int i = 0; i < n; i++) {
			d[i] = scan.nextInt();;
		}
		for(int i = 0; i < n -1; i++) {
			for(int j = i + 1; j < n; j++) {
				ans +=  d[i] * d[j];
			}
		}
		System.out.println(ans);
	}
}
/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