import java.util.*;
public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		// 整数の入力
        int n = sc.nextInt();
        int d[] = new int[n];
		int ans = 0;
        for(int i = 0; i < n;i++){
			d[i] = sc.nextInt();
        }
      
        for(int i = 0; i < n;i++){
			for(int j = i + 1; j < n;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