import java.util.*;

public class Main {
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.nextLine());
        String line = sc.nextLine();
        String[] line1 = line.split(" ");
        int[] ary = new int[n];
        for(int i = 0; i < n; i++)  ary[i] = Integer.parseInt(line1[i]);
        int count = 0;
        for(int i = 0; i < n; i++){
            for(int j = i + 1; j < n; j++){
                count += ary[i] * ary[j];
            }
        }
        System.out.println(count);
    }
}
/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