import java.io.*;
import java.util.*;

class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int N = Integer.parseInt(sc.next());
    
    int[] d = new int[N];
    for (int i = 0; i < N; i++) {
      d[i] = Integer.parseInt(sc.next());
    }
    
    int sum = 0;
    for (int i = 0; i < N-1; 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