题目描述
Akari has n kinds of flowers, one of each kind.
She is going to choose one or more of these flowers to make a bouquet.
However, she hates two numbers a and b, so the number of flowers in the bouquet cannot be a or b.
How many different bouquets are there that Akari can make?
Find the count modulo (10^9 + 7).
Here, two bouquets are considered different when there is a flower that is used in one of the bouquets but not in the other bouquet.
-----Constraints-----
- All values in input are integers.
- $2 \leq n \leq 10^9$
- $1 \leq a < b \leq \textrm{min}(n, 2 \times 10^5)$
-----Input-----
Input is given from Standard Input in the following format:
n a b
-----Output-----
Print the number of bouquets that Akari can make, modulo (10^9 + 7). (If there are no such bouquets, print 0.)
-----Sample Input-----
4 1 3
-----Sample Output-----
7
In this case, Akari can choose 2 or 4 flowers to make the bouquet.
There are 6 ways to choose 2 out of the 4 flowers, and 1 way to choose 4, so there are a total of 7 different bouquets that Akari can make.
输入格式
见题面
输出格式
见题面