import java.util.*;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		int L = sc.nextInt(), K = sc.nextInt();
		
		int cnt = 0;
		
		while(true) {
			cnt+=K * 2;
			
			if(cnt >= L) {
				System.out.println(cnt / 2 - K);
				break;
			}
		}
	}
}