// Program 28: Hamming Distance of Binary Strings

import java.util.*;
class HammingDistance {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String a = sc.next();
        String b = sc.next();
        if (a.length() != b.length()) {
            System.out.println("Unequal lengths");
            return;
        }
        int distance = 0;
        for (int i = 0; i < a.length(); i++) {
            char x = a.charAt(i);
            char y = b.charAt(i);
            if ((x != '0' && x != '1') || (y != '0' && y != '1')) {
                System.out.println("Invalid binary input");
                return;
            }
            if (x != y) {
                distance++;
            }
        }
        System.out.println("Hamming distance = " + distance);
    }
}