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 PyPy3 (2.4.0) (judge verdict: Accepted)



n,a,b = map(int,input().split())
from functools import reduce
def mod_comb(n,r,mod=10**9+7):
    num=reduce(lambda x,y:x*y%mod,range(n,n-r,-1))
    den=reduce(lambda x,y:x*y%mod,range(1,r+1))
    return num*pow(den,mod-2,mod)%mod
mod = 10**9+7
bi = [(2**i)%mod for i in range(30) if 2**i<=n]
ans = 1
s = list(reversed(bin(n)[2:]))
for i in range(len(s)):
    ans *= 2**(bi[i]*int(s[i]))
    ans %= mod
ans = (ans+mod-1)%mod
ans = (ans-mod_comb(n,a)-mod_comb(n,b))%mod
print(ans)