using System; using System.Collections; using System.Collections.Generic; using System.Linq; using System.Text; class TEST{ static void Main(){ Sol mySol =new Sol(); mySol.Solve(); } } class Sol{ public void Solve(){ if(K == 0 || K > N){ Console.WriteLine(0); return; } var X = new long[]{K,N-K+1}.Distinct().Count(); Console.WriteLine(N-X); } long N,K; public Sol(){ var d = rla(); N = d[0]; K = d[1]; } static String rs(){return Console.ReadLine();} static int ri(){return int.Parse(Console.ReadLine());} static long rl(){return long.Parse(Console.ReadLine());} static double rd(){return double.Parse(Console.ReadLine());} static String[] rsa(char sep=' '){return Console.ReadLine().Split(sep);} static int[] ria(char sep=' '){return Array.ConvertAll(Console.ReadLine().Split(sep),e=>int.Parse(e));} static long[] rla(char sep=' '){return Array.ConvertAll(Console.ReadLine().Split(sep),e=>long.Parse(e));} static double[] rda(char sep=' '){return Array.ConvertAll(Console.ReadLine().Split(sep),e=>double.Parse(e));} }