program tax
  implicit none
  integer(8) :: a, b
  integer(8) :: x
  read(*,*) a, b

  x = 10 * b
  
  if ( x * 0.08 >= a .and. x * 0.08 < a + 1 ) then
     write(*,*) x
  else if ( x * 0.08 >= a + 1  ) then
     write(*,*) -1
  else
     write(*,*) int(real(a / 0.08, 8),8)
  end if
  stop
end program tax
