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