import java.util.Scanner;
 
public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int tako[] = new int[N];
    for(int i=0; i<N; i++){
        tako[i] = sc.nextInt();
    }


    int sumTako = 0;
    for(int i=0; i<N; i++){
        for (int j=i+1; j<N; j++)
            sumTako += (tako[i]*tako[j]);
    }

    System.out.println(sumTako);

    }
}
/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