Problem:
Score : 400 points

Problem Statement

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 1

4 1 3

Sample Output 1

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.

Sample Input 2

1000000000 141421 173205

Sample Output 2

34076506

Print the count modulo (10^9 + 7).

Solution in Python (3.4.3) (judge verdict: Accepted)

n, a, b = map(int, input().split())
mod = 10**9+7
def Pow(a, n):
    fac = a
    res = 1
    for i in range(len(bin(n)[2:])):
        if n&1:
            res = res*fac%mod
        fac = fac*fac%mod
        n = n>>1
    return res
cnt = Pow(2, n) - 1
x = [0] * (b+1)
for i in range(1,b+1):
    x[i] = Pow(i, mod-2)
cmb = [0] * (b+1)
cmb[1] = n
for i in range(2, b+1):
    cmb[i] = cmb[i-1]*(n-i+1)%mod*x[i]%mod
cnt -= cmb[a]+cmb[b]
print(cnt%mod)