백준 #18301번 Rats

import java.util.Scanner;
public class Main {
	public static void main(String[] args) {
		
		Scanner sc = new Scanner(System.in);
		
		int n1 = sc.nextInt();
		int n2 = sc.nextInt();
		int n12 = sc.nextInt();
		
		System.out.println((n1 + 1) * (n2 + 1) / (n12 + 1) - 1);
		sc.close();
	}
}