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 PHP7 (7.0.15) (judge verdict: Accepted)

<?php
define("MOD", 10**9+7);
list($n,$a,$b)=explode(" ",trim(fgets(STDIN)));
$i=1;
$ans=powerm(2,$n)-1;
$ans-=nCm($n,$a);
$ans-=nCm($n,$b);
while($ans<0){
  $ans+=MOD;
}
echo $ans;

//yoichiro's func
//掛け算
function mulm($a, $b){
    return ($a * $b) % MOD;
}

//割り算
function divm($a, $b){
    return mulm($a, powerm($b, MOD - 2));
}

//累乗
function powerm($a, $b){
    if($b == 0)return 1;
    if($b & 1)return (powerm($a, $b>>1)**2 % MOD) * $a % MOD;
    return powerm($a, $b>>1)**2 % MOD;
}

//階乗
function factorialm($a){
    if($a == 0)return 1;
    return factorialm($a - 1) * $a % MOD;
}

//順列
function nPm($n, $m){
    if($m == 0)return 1;
    return nPm($n, $m - 1) * ($n - $m + 1) % MOD;
}

//組み合わせ
function nCm($n, $m){
    $m = min($m, $n-$m);
    return divm(nPm($n, $m), factorialm($m));
}