import java.util.Scanner

object Main {

  def main(args: Array[String]): Unit = {
    val sc = new Scanner(System.in)
    val s = sc.next().toCharArray
    val t = sc.next().toCharArray

    var cnt = 0
    (0 until s.length).foreach { i =>
      if (s(i) != t(i)) cnt += 1
    }

    println(cnt)
  }
}
