import std.algorithm, std.conv, std.range, std.stdio, std.string;

void readV(T...)(ref T t){auto r=readln.splitter;foreach(ref v;t){v=r.front.to!(typeof(v));r.popFront;}}
T[] readArray(T)(size_t n){auto a=new T[](n),r=readln.splitter;foreach(ref v;a){v=r.front.to!T;r.popFront;}return a;}
T[] readArrayM(T)(size_t n){auto a=new T[](n);foreach(ref v;a)v=readln.chomp.to!T;return a;}

void main()
{
  int n, k; readV(n, k);
  if (k == 0 || k > n)
    writeln(0);
  else if (n&1 && (n+1)/2 == k)
    writeln(n-1);
  else
    writeln(n-2);
}