implicit none
integer(8) L,K,Y,C
read(*,*) L,K
C=L/K
if (mod(L,K)==0) then
   if (mod(C,2)==0) then
       Y=(L/2)-K
       write(*,*) Y
   else
       Y=(C/2)*K
       write(*,*) Y
   end if
else
   Y=(C/2)*K
   write(*,*) Y
end if
end