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 C# (Mono 4.6.2.0) (judge verdict: Time Limit Exceeded)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

class D {
    static long mod = 1000000007;
    static void Main(string[] args) {
        var st = Console.ReadLine().Split().Select(long.Parse).ToArray();
        long n = st[0];
        long a = st[1];
        long b = st[2];

        long ans = 0;

        ulong factorial_n = FactorialMod((ulong)n);

        var factorialModInvTable = new long[n + 1];

        for (long i = 0; i < n + 1; i++) {
            factorialModInvTable[i] = FactorialModInv(i);
        }

        for (long i = 1; i <= n; i++) {
            if (i == a || i == b) {
                continue;
            }

            ans += (long)factorial_n * factorialModInvTable[n - i] % mod * factorialModInvTable[i] % mod;
            ans %= mod;
        }
        Console.WriteLine(ans);
    }

    static ulong FactorialMod(ulong a) {

        if (a <= 1) {
            return 1;
        } else {
            return a * FactorialMod(a - 1) % (ulong)mod;
        }
    }

    static long ModInv(long a) {
        if (a <= 1) {
            return 1;
        } else {
            long ret = -(ModInv(mod % a) * (mod / a)) % mod;
            if (ret < 0) {
                ret += mod;
            }

            return ret;
        }
    }

    static long FactorialModInv(long a) {
        if (a <= 1) {
            return 1;
        } else {
            return ModInv(a) * FactorialModInv(a - 1) % mod;
        }
    }
}