- Time limit: 1.00 s
- Memory limit: 512 MB
Your task is to calculate the number of valid bracket sequences of length n when a prefix of the sequence is given.
The first input line has an integer n.
The second line has a string of k characters: the prefix of the sequence.
Print the number of sequences modulo 10^9+7.
- 1 \le k \le n \le 10^6
Explanation: There are two possible sequences: