import java.util.Scanner;
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 a = 0;
    for(int i=0; i<n; i++) {
    	d[i] = sc.nextInt();
    	a += d[i]*d[i];
    }
    int all = 0;
    for(int i=0; i<n; i++) {
    	for(int j=0; j<n; j++) {
    		all += d[i]*d[j];
     	}
    }
    System.out.print((all-a)/2);
    }
}

/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