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: Wrong Answer)

def pow_k(x, n):
    """
    O(log n)
    """
    if n == 0:
        return 1

    K = 1
    while n > 1:
        if n % 2 != 0:
            K *= x
        x *= x
        n //= 2

    return K * x

mod = 10 ** 9 + 7



n, a, b = map(int,input().split())

m = min(n, max(a, b) + 10)
inv = [0,1] + [0 for _ in range(m)]
for i in range(2,m+2):
    inv[i] = -inv[mod%i] * (mod // i) % mod

def comb(n,r):
    ret = 1
    for i in range(r):
        ret *= n-i
        ret *= inv[i+1]
        ret %= mod
    return ret



nca = comb(n, a) #comb(n, a, mod)#factorial(n) // factorial(a) // factorial(n-a)
ncb = comb(n, a) #comb(n, b, mod)#factorial(n) // factorial(b) // factorial(n-b)

ans = pow_k(2, n)

ans -= 1 + nca + ncb
ans %= mod
print(ans)