import java.util.Scanner;
class Main {
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    int[] a = new int[n];
    for(int i=0; i < n; i++) {
      a[i] = sc.nextInt();
    }
    int S1 =0;
    int S2 = 0;
    for( int i =  0; i < n; i++) {
      S1 += a[i]*a[i];
    }
    for(int i = 0; i< n;i++) {
      S2 += a[i];
    }
    System.out.println(((S2)*(S2)-S1)/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