import java.util.*;

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

/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