// AtCoder Beginner Contest 143b
import java.util.Scanner;

public class Main {
    public static void main(String args[]) {

        // Input
        Scanner sc = new Scanner(System.in);

        int kosuu = sc.nextInt();
        int[] arrayOishi = new int[kosuu];
        for ( int i = 0; i < arrayOishi.length; i++ ) {
            arrayOishi[i] = sc.nextInt();
        }

        // Calc
        int result = 0;
        for ( int j = 0; j < kosuu; j++ ) {
           for ( int k = j + 1; k < kosuu; k++ ) {
              result += arrayOishi[j] * arrayOishi[k];
           }
        }

        // Output
        System.out.println(result);

        sc.close();
    }
}
/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