import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        List<Integer> d = new ArrayList<>();
        for (int i=0; i<N; i++) {
            d.add(sc.nextInt());
        }
        
        int count = 0;
        for (int s=0; s<d.size()-1; s++) {
            for (int s2=s+1; s2<d.size(); s2++) {
                count += d.get(s)*d.get(s2);
            }
        }
        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