import java.util.*;
public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    String a = sc.next();
    String b = sc.next();
    
    int ans = 0;
    
    if(a.length() != b.length()) {
      System.out.println("-1");
    }
    
    for(int i=0; i<a.length(); i++) {
      if(a.charAt(i) != b.charAt(i)) {
        ans++;
      }
    }
    System.out.println(ans);
  }
} import java.util.*; util.* java public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    String a = sc.next();
    String b = sc.next();
    
    int ans = 0;
    
    if(a.length() != b.length()) {
      System.out.println("-1");
    }
    
    for(int i=0; i<a.length(); i++) {
      if(a.charAt(i) != b.charAt(i)) {
        ans++;
      }
    }
    System.out.println(ans);
  }
} Main public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    String a = sc.next();
    String b = sc.next();
    
    int ans = 0;
    
    if(a.length() != b.length()) {
      System.out.println("-1");
    }
    
    for(int i=0; i<a.length(); i++) {
      if(a.charAt(i) != b.charAt(i)) {
        ans++;
      }
    }
    System.out.println(ans);
  } main {
    Scanner sc = new Scanner(System.in);
    String a = sc.next();
    String b = sc.next();
    
    int ans = 0;
    
    if(a.length() != b.length()) {
      System.out.println("-1");
    }
    
    for(int i=0; i<a.length(); i++) {
      if(a.charAt(i) != b.charAt(i)) {
        ans++;
      }
    }
    System.out.println(ans);
  } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) String a = sc.next(); a sc.next() sc.next sc String b = sc.next(); b sc.next() sc.next sc int ans = 0; ans 0 if(a.length() != b.length()) {
      System.out.println("-1");
    } a.length() != b.length() a.length() a.length a b.length() b.length b {
      System.out.println("-1");
    } System.out.println("-1") System.out.println System.out System System.out "-1" for(int i=0; i<a.length(); i++) {
      if(a.charAt(i) != b.charAt(i)) {
        ans++;
      }
    } int i=0; int i=0; i 0 i<a.length() i a.length() a.length a i++ i++ i {
      if(a.charAt(i) != b.charAt(i)) {
        ans++;
      }
    } {
      if(a.charAt(i) != b.charAt(i)) {
        ans++;
      }
    } if(a.charAt(i) != b.charAt(i)) {
        ans++;
      } a.charAt(i) != b.charAt(i) a.charAt(i) a.charAt a i b.charAt(i) b.charAt b i {
        ans++;
      } ans++ ans System.out.println(ans) System.out.println System.out System System.out ans String[] args args public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    String a = sc.next();
    String b = sc.next();
    
    int ans = 0;
    
    if(a.length() != b.length()) {
      System.out.println("-1");
    }
    
    for(int i=0; i<a.length(); i++) {
      if(a.charAt(i) != b.charAt(i)) {
        ans++;
      }
    }
    System.out.println(ans);
  }
} public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    String a = sc.next();
    String b = sc.next();
    
    int ans = 0;
    
    if(a.length() != b.length()) {
      System.out.println("-1");
    }
    
    for(int i=0; i<a.length(); i++) {
      if(a.charAt(i) != b.charAt(i)) {
        ans++;
      }
    }
    System.out.println(ans);
  }
} Main
import java.util.*;
public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    String a = sc.next();
    String b = sc.next();
    
    int ans = 0;
    
    if(a.length() != b.length()) {
      System.out.println("-1");
    }
    
    for(int i=0; i<a.length(); i++) {
      if(a.charAt(i) != b.charAt(i)) {
        ans++;
      }
    }
    System.out.println(ans);
  }
}
