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: Time Limit Exceeded)

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

#half_expo(a, n, mod) = (a**n) % mod
#二分累乗法
def half_expo_mod(a, n, mod):
    nbin = list(bin(n))
    nbin = nbin[3:]
    ans = a
    for j in nbin:
        if j == '0':
            ans **= 2
        else:
            ans = (ans ** 2) * a
        ans %= mod
    if n == 0: #n=0の時の処理
        ans = 1
    return ans

#comb_mod(n, r, mod) = nCr % mod
def comb_mod(n, r, mod):
    ans = 1
    if r <= n/2:
        for i in range(n-r+1, n+1):
            ans *= i
            ans %= mod
        for i in range(1, r+1):
            ans *= half_expo_mod(i, mod-2, mod)
            ans %= mod
    else:
        for i in range(r+1, n+1):
            ans *= i
            ans %= mod
        for i in range(1, n-r+1):
            ans *= half_expo_mod(i, mod-2, mod)
            ans %= mod
    return ans

ans = half_expo_mod(2, n, 10**9+7) - comb_mod(n, a, 10**9+7) - comb_mod(n, b, 10**9+7) - 1
if ans < 0:
    ans += 10**9 + 7
if ans < 0:
    ans += 10**9 + 7

print(int(ans))