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 result = 0;
      for (int i = 0; i < N; ++i) {
        for (int j = i + 1; j < N; ++j) {
          result += d[i] * d[j];
        }
      }
      
      
      System.out.println(result);
    }

}
/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