import java.util.Scanner;

public class Main {
    public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int[] d = new int[n];
        for (int i=0; i<n; i++) d[i] = sc.nextInt();
        int count = 0;
        for (int i=0; i<n; i++) {
            for (int j=i+1; j<n; j++) {
                count += d[i] * d[j];
            }
        }
        System.out.print(count);
    }
}
/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