using System;
using System.Linq;
class Program {
    static void Main() {
        int[] nk = Console.ReadLine().Split().Select(int.Parse).ToArray();
        int n = nk[0], k = nk[1], ans = 0;

        if (0 < k && k <= n) {
            if (2 * k == n + 1) ans = n - 1;
            else ans = n - 2;
        }
        Console.WriteLine(ans);
    }
}