using System;
using System.Linq;

class No521{
    static void Main(){
        var count=Console.ReadLine().Split(' ').Select(x=>Int32.Parse(x)).ToArray();
        var ans=0;
        if(count[1]>=1&&count[0]>=count[1]&&count[0]!=1){
            if(count[0]%2==1&&count[0]/2+1==count[1])ans=count[0]-1;
            else ans=count[0]-2;
        }
        Console.WriteLine(ans);
    }
}