import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        ArrayList<Integer> takoyaki = new ArrayList<>();
        for (int i = 0; i < n; i++) {
            takoyaki.add(sc.nextInt());
        }
        sc.close();

        int ans = 0;
        for(int i = 0; i < n - 1; i++){
            for(int j = i + 1; j < n; j++){
                ans += takoyaki.get(i) * takoyaki.get(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