source_code: import java.util.Scanner;
  
  public class Main {
  	public static void main(String[] args) {
  		Scanner sc = new Scanner(System.in);
  
  		long n = sc.nextLong();
  		long k = sc.nextLong();
  		System.out.println(solve(n, k));
  
  		sc.close();
  	}
  
  	static long solve(long n, long k) {
  		return Math.max(0, Math.min(n, k - 1) - ((k + 2) / 2) + 1);
  	}
  }
  