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 H = 0;
		for(int i = 0; i < N; i++) {
			d[i] = sc.nextInt();
		}
		int j = 0;
		for(int i = 0; i <((N*(N-1))/2); i++) {
			j = i+1; 
			while(j < N) {
				int h = d[i] * d[j];
				H = h + H;
				j++;
			}
		}
		System.out.println(H);
	}
}
/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