import java.util.*;
import java.lang.Math;

class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();

    ArrayList<Integer> d = new ArrayList<Integer>(n);
    for (int i = 0; i < n; i++) {
      d.add(sc.nextInt());
    }
    
    int c = 0;
    for (int i = 0; i < n; i++) {
      for (int j = i+1; j < n; j++) {
        c += d.get(i) * d.get(j);
      }
    }
    System.out.println(c);
  }
}
/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