program alter_altar
    implicit none
    integer :: n, m = 0, l = 1, r
    character(200000) :: s
    read(*,*) n
    read(*,*) s
    r = n
    do
      do while (l <= n .and. s(l:l) == 'R')
        l = l + 1
      end do
      do while (r >= 1 .and. s(r:r) == 'W')
        r = r - 1
      end do
      if (l >= r) exit
      m = m + 1
      s(l:l) = 'R'
      s(r:r) = 'W'
    end do
    write(*,'(i0)') m
  end program alter_altar