program Project2;

{$APPTYPE CONSOLE}

uses
  SysUtils;

Var N,M,g,k,l:int64;
begin
assign(input,'problem.in');
reset(input);
close(input);
readln(N,M);

assign(output,'problem.out');
rewrite(output);
g:=N-1;
if (N>0) then k:=2*g+1;
l:=2*N div (M div 2)-1;
writeln(l);
readln;
close(output);
  { TODO -oUser -cConsole Main : Insert code here }
end.