import java.util.Scanner;
public class Main{
	public static void main(String[] args) {
		Scanner sc=new Scanner(System.in);
		int n=sc.nextInt();
		int a[]=new int[n];
		int sum=0;
		for(int i=0;i<n;i++) {
			a[i]=sc.nextInt();
			sum+=a[i];
		}
		//int b[][]=new int[n][n];
		int ans=0;
		for(int i=0;i<n;i++) {
			ans+=a[i]*(sum-a[i]);
		}
		System.out.println(ans/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