import java.util.*;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N =sc.nextInt();
		int[] d = new int[N];
		int sum=0;
		
		for(int i=0;i<d.length;i++) {
			d[i]=sc.nextInt();
		}
		
		for(int i=0;i<(d.length-1);i++) {
			for(int j=i+1;j<d.length;j++) {
				sum+=d[i]*d[j];
			}
		}
		
		System.out.println(sum);
		
	}
}

/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