import java.util.ArrayList;
import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.next());
        ArrayList<Integer> d = new ArrayList<>();
        for (int i = 0; i < n; i++) {
            d.add(Integer.parseInt(sc.next()));
        }
        int answer = 0;
        for (int i = 0; i < (n - 1); i++) {
            for (int j = 1 + i; j < n; j++) {
                answer += d.get(i) * d.get(j);
            }
        }
        System.out.println(answer);
        sc.close();
    }
}
/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