import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        int n,sum = 0;
        ArrayList<Integer> list = new ArrayList<Integer>();
        
        n = sc.nextInt();
        
        for(int i = 0; i < n; i++){
            list.add(sc.nextInt());
        }
        
        while(list.size() > 0){
            for(int i = 1; i < list.size(); i++){
                sum += list.get(0) * list.get(i); 
            }
            list.remove(0);
        }
        
        System.out.println(sum);
        
    }
}
/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