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: Time Limit Exceeded)

def mod_comb(a,b,mod):
    #print(a,b)
    if a-b < b: return mod_comb(a,a-b,mod)
    ans_mul = 1
    ans_div = 1
    for i in range(b):
        ans_mul *= a-i
        ans_div *= i+1
        ans_mul %= mod
        ans_div %= mod
    return (ans_mul * modpow(ans_div, mod-2, mod)) % mod

def modpow(a,b,mod):
    if b == 0: return 1
    if b % 2 == 0:
        h = modpow(a, b//2, mod)
        return (h * h) % mod
    else:
        return (a * modpow(a, b-1, mod)) % mod


words = lambda t : list(map(t, input().split()))
n,a,b = words(int)


top = 1
btm = 1
ans = 0
mod = 1000000007
for i in range(1,n+1):
    top *= n - i + 1
    top %= mod
    btm *= modpow(i, mod-2, mod)
    btm %= mod
    if i != a and i != b:
        ans += top * btm
        ans %= mod

print(ans)