import scala.math.pow

object Main extends App {
  val sc = new java.util.Scanner(System.in)
  var s = sc.next()
  var t = sc.next()
  var ans = 0
  for(i <- 0 until s.length){
    if (s(i)!= t(i)){
      ans += 1
    }
  }
  println(ans)
}
