defmodule Main do
  def main do
    ss = input(:strs) 
    ts = input(:strs)
    
    diffs = 
      for {s, t} <- Enum.zip(ss, ts) do
        unless s === t do
          1
        else
          0
        end
      end
    
    diffs
    |> Enum.sum 
    |> IO.puts
  end
  
  defp input(type \\ :str) do
    s = IO.gets("") |> String.trim()
    
    case {type, s} do
      {:int, s}  -> s |> String.to_integer()
      {:ints, s} -> s |> String.split("") |> Enum.map(&String.to_integer(&1))
      {:str, s}  -> s
      {:strs, s} -> s |> String.split("")
      _ -> raise "TypeError"
    end
  end
end