package main

import (
	"bufio"
	"fmt"
	"os"
)

func main() {
	in := bufio.NewReader(os.Stdin)
	var a, b string
	fmt.Fscan(in, &a, &b)

	d := 0
	for i := 0; i < len(a); i++ {
		if a[i] != b[i] {
			d++
		}
	}
	fmt.Println(d)
}