program probB
  implicit none
  character(len=200000) :: S, T
  integer :: i, c
  read(*,*) S, T

  c = 0
  do i = 1, len_trim(S)
     if (S(i:i) /= T(i:i)) then
        c = c + 1
     end if
  end do
  write(*,'(i0)') c
  stop
end program probB
