import java.util.Scanner;

public class Main{
	public static void main(String[] args){
		Scanner scan = new Scanner(System.in);
		int N = scan.nextInt();
		int d[] = new int[N];
		for(int i=0;i<N;i++){
			d[i] = scan.nextInt();
		}
		
		int Ans = 0;
		for(int i=0;N>i;i++){
			for(int j=1+i;N>j;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