implicit none
integer(8) L,K,Y,C
read(*,*) L,K
C=L/K
if (L==2*K) then
    write(*,*) "0"
else if (mod(L,K)==0) then
    Y=((C/2)-1)*K
    write(*,*) Y
else    
    Y=(C/2)*K
    write(*,*) Y
end if
end