import java.util.*;
public class Main {
    public static void main(String args[]){
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int opt = 0;
        ArrayList<Integer> D = new ArrayList<>();
        for(int i=0;i<N;i++) D.add(sc.nextInt());
        for(int i=0;i<N-1;i++) for(int j=i+1;j<N;j++) opt+=D.get(i)*D.get(j);
        System.out.println(opt);
    }
}

/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