package quki.algorithm.string;

import java.util.ArrayList;

import java.util.Scanner;

public class BOJ10453 {

public static void main(String[] args) {

Scanner sc = new Scanner(System.in);

int N = sc.nextInt();

sc.nextLine();

while (N-- > 0) {

String line[] = sc.nextLine().split(" ");

String A = line[0];

String B = line[1];

if (A.length() != B.length()) {

System.out.println(-1);

} else if (A.equals(B)) {

System.out.println(0);

} else {

ArrayList<Integer> aList = new ArrayList<>();

ArrayList<Integer> bList = new ArrayList<>();

for (int i = 0; i < A.length(); i++) {

char a = A.charAt(i);

char b = B.charAt(i);

if (a == 'a') {

aList.add(i);

}

if (b == 'a') {

bList.add(i);

}

}

if (aList.size() != bList.size()) {

System.out.println(-1);

} else {

int ans = 0;

for (int i = 0; i < aList.size(); i++) {

ans += Math.abs(aList.get(i) - bList.get(i));

}

System.out.println(ans);

}

}

}

}

}