program answer
  implicit none

  character(len=2*100000) :: S,T
  integer :: n, i, count

  read(*,*) S
  read(*,*) T

  n=len_trim(S)

  count = 0
  do i = 1, n
     if(S(i:i)/=T(i:i)) then
        count = count + 1
     end if
  end do

  write(*,*) count

  stop
 end program answer