import java.util.Scanner;
class Main {
	public static void main(String[] args){

		Scanner sc = new Scanner(System.in);

      	int X = 0;
      
		int N = sc.nextInt();
      
		int [] M;
		M = new int[N];

		for (int i = 0; i < N; i++) {
			M[i] = sc.nextInt();
		}
      
		for (int i = 0; i < N - 1; i++) {
        	for (int j = i + 1; j < N ; j++) {
        		X = X + M[i] * M[j];
        	}
        }
      	
      	System.out.print(X);
		return;
	}
}
/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