import java.io.*;
import java.util.Scanner;

public class Main {
	public static void main(String[] args) throws IOException {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(System.out));
		Scanner sc = new Scanner(System.in);
		
		int n = sc.nextInt();
		int[] arr = new int[n];
		
		for (int i = 0; i < arr.length; i++) {
			arr[i] = sc.nextInt();
		}
		int total = 0;
		for (int i = 0; i < arr.length; i++) {
			for (int j = i+1; j < arr.length; j++) {
				total += arr[i]*arr[j];
			}
		}
		System.out.println(total);
		bw.close();
	}

}

/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