import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] d = new int[n];
        for(int i = 0; i < n; i++)
        	d[i] = sc.nextInt();

        long hp = 0;

        for(int i = 0; i < n-1; i++) {
        	for(int j = i+1; j < n; j++) {
        		hp += d[i] * d[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