import java.util.Scanner;

public class Main{
    static boolean eq(String a, String b){
        if(a.length() != b.length()) return false;
        for(int i=0; i<a.length(); i++){
            if(a.charAt(i) != b.charAt(i)) return false;
        }
        return true;
    }
    public static void main(String[] arg){
        Scanner sc = new Scanner(System.in);
        int t = sc.nextInt();
        while(t-->0){
            String a = sc.next();
            String b = sc.next();
            if(!eq(a,b)) System.out.println(Math.max(a.length(), b.length()));
            else System.out.println("-1");
        }
        sc.close();
    }
}