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문제

Your friend and you took the same true/false exam. You know your answers, your friend’s answers, and the number of your friend’s answers that were correct.

Compute the maximum possible score you could have gotten

입력

The first line contains a single integer k, the number of correct answers on your friend’s exam.

The second line contains a string of characters, the answers you wrote down. Each letter is either a ‘T’ or an ‘F’. The length of the string is the number n of exam questions.

The third line also contains a string of n characters, the answers your friend wrote down. Each letter is either a ‘T’ or an ‘F’.

Bounds are 0 ≤ k ≤ n ≤ 1 000; 1 ≤ n.

출력

The output is one line containing the maximum number of questions you could have gotten correct.

예제 입력 1

3
FTFFF
TFTTT

예제 출력 1

2

예제 입력 2

6
TTFTFFTFTF
TTTTFFTTTT

예제 출력 2

9