import java.util.*;

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

    int sum = 0;
    int v1 = 0;
    int v2 = 0;

    for(int i=0; i<n;i++){
        v1 = array[i];
      for(int j=i+1; j<n; j++){
        v2 = array[j];
        sum += v1 * v2;
      }
    }

    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