object Main {
  def main(args: Array[String]): Unit = {
    import scala.io.StdIn.readLine
    val datS = readLine
    val datT = readLine

    var ans = 0;
    (0 until datS.length) foreach { i =>
      if (datS(i) != datT(i)) ans += 1
    }

    println(ans)
  }
}
