Brackets all over - problem not clear

#1

I can’t understand the problem. What are strings a and b, and what is the use of ‘n’.
It would be great if you can explain with an example.

#2

n is the size of the string which will be after adding a + s + b.
a and b are the strings you have to add to original string of size m to make it to size n where n = a+s+b and count no of ways it can be done modulo 10^9 + 7 , if answer exists otherwise print -1.
For example :
if n = 4 , m = 2 and s = “((”.
as you have to make this string of length 4 and satisfy the conditions of the problem , a = “” and b = “))” . In this way a + s+ b becomes 4 and there is only 1 way of doing it so answer is 1

#3

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