Solve the programming task below in a Python markdown code block.
Given a string A of length N consisting of lowercase English alphabet letters. 

You are allowed to perform the following operation on the string A any number of times: 

Select a non-empty [subsequence] S of the array [1,2,3,\ldots,N] and any lowercase English alphabet \alpha;
Change A_{i} to \alpha for all i \in S.

Find the minimum number of operations required to convert A into a given string B of length N consisting of lowercase English alphabet letters. 

------ Input Format ------ 

- The first line of input contains a single integer T, denoting the number of test cases. The description of T test cases follow.
- The first line of each test case contains an integer N - the length of the string A and B.
- The second line of each test case contains the string A. 
- The third line of each test case contains the string B. 

------ Output Format ------ 

For each test case, output the minimum number of operations required to convert string A into string B. 

------ Constraints ------ 

$1 ≤ T ≤ 10^{4}$
$1 ≤ N ≤ 10^{5}$
- Sum of $N$ over all test cases does not exceed $10^{5}$.

----- Sample Input 1 ------ 
3
2
ab
cd
3
aaa
bab
4
abcd
aaab

----- Sample Output 1 ------ 
2
1
2

----- explanation 1 ------ 
Test case 1:
- In the first operation, we can choose $S = \{1\}$ and $\alpha =$ c. After this operation, $A$ will become cb.
- In the second operation, we can choose $S = \{2\}$ and $\alpha =$ d. After this operation, $A$ will become cd. 

Test case 2: 
- In the first operation, we can choose $S = \{1, 3\}$ and $\alpha =$ b. After this operation, $A$ will become bab.

Test case 3:
- In the first operation, we can choose $S = \{2, 3\}$ and $\alpha =$ a. After this operation, $A$ will become aaad.
- In the second operation, we can choose $S = \{4\}$ and $\alpha =$ b. After this operation, $A$ will become aaab.