program million_change
    implicit none
    character(200000) :: S
    character(200000) :: T
    integer i
    integer :: k = 0
    read *, S
    read *, T
    do i = 1, len_trim(S)
        if(S(i:i) /= T(i:i))then
            S(i:i) = T(i:i)
            k = k + 1
        end if
    end do
    print *, k
end program