import java.util.Scanner;

fun main (args : Array<String>) {
  val input = Scanner(System.`in`)
  
  while (input.hasNext()) {
    val S = input.next()
    val T = input.next()
    var operationCount = 0;
    
    for (i in 0..S.length - 1) {
      if (S[i] != T[i]) {
        ++operationCount
      }
    }
    
    println(operationCount)
  }
}