import java.util.Scanner;
import java.util.List;
import java.util.ArrayList;
import java.util.Comparator;
import java.util.Arrays;
import java.util.Collections;
import java.util.*;
class Main{
  public static void main(String[] args){
    Scanner scan = new Scanner(System.in);
    int n = scan.nextInt();
    int d[] = new int[n];
    int sum = 0;
    for (int i = 0;i < n;i++) {
      d[i] = scan.nextInt();
    }
    for (int i = 0;i < n;i++) {
      for (int j = i + 1;j < n;j++) {
        sum += d[i] * d[j];
      }
    }
    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