import java.util.*;

public class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int d[]= new int[N];
    int sum = 0;
    
    for(int i = 0; i<N; i++){
      d[i]=sc.nextInt();
      sum += d[i];
    }
    
    int ans = 0;
    for(int i = 0; i<N; i++){
      ans += d[i]*(sum-d[i]);
    }
    
    System.out.println(ans/2);
  }
}
/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