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)

def pow_self(x,n):
    ans = 1
    while n>0:
        if n&1 == 1:
            ans = (ans*x)%(10**9+7)
            
        x = (x*x)%(10**9+7)
        n >>= 1
    return ans

  
n,a,b = map(int, input().split())        
X,Y = 1,1
for i in range(b):
    X *= n-i
    X %= 10**9+7
    Y *= i+1
    Y %= 10**9+7

    if a-1 == i:
        Xa,Ya = X,Y

comb_a = (Xa*pow_self(Ya,10**9+5))%(10**9+7)
comb_b = (X*pow_self(Y,10**9+5))%(10**9+7)

result = pow_self(2,n)-1-comb_a-comb_b
while result <0:
    result += 10**9+7
print(result)