import java.util.*

fun main(args: Array<String>) {
    val scan = Scanner(System.`in`)
    var a=scan.nextLine()
    var b = scan.nextLine()
    var ans = 0
    for(i in 0..a.length-1)
    {
        if(a[i]!=b[i])
            ans++
    }
    println(ans)
}
