import java.util.*;

public class Main {
  public static void main(String[] args ) throws Exception {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int answer = 0;

    List<Integer> ds = new ArrayList<>();
    for (int z = 0; z < N; z++) {
        ds.add(sc.nextInt());
    }
    for(int i = 0; i < N - 1; i++) {
        for(int j = i+1; j < N; j++) {
            answer += ds.get(i) * ds.get(j);
        }
    }
    System.out.println(answer);
  }
}

/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