import java.util.Scanner;

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

    		for (int i=0; i<N-1; i++) {
    			for (int j=i+1; j<N; j++) {
    				sum += d[i] * d[j];
    			}
    		}

    		System.out.println(sum);

    	} finally {
    		sc.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