import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner in = new Scanner(System.in);
		int n = Integer.parseInt(in.next());
		int m = Integer.parseInt(in.next());
		int all = (n + m) * (m + n - 1) / 2;
		int odd = n * m;
		int even = all - odd;
		
		System.out.println(even);
	}
}	
/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