program main
    implicit none
    integer i,n,c
    character s*200000, t*200000
    read(*,*) s
    read(*,*) t
    n = len_trim(s)
    c = 0
    do i = 1,n
        if(s(i:i) /= t(i:i)) then
            c = c + 1
        end if
    end do
    write(*,*) c

end program
