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++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <bits/stdc++.h>

using namespace std;

template <typename T>
T inverse(T a, T m) {
  T u = 0, v = 1;
  while (a != 0) {
    T t = m / a;
    m -= t * a;
    u -= t * v;
    swap(a, m);
    swap(u, v);
  }
  assert(m == 1);
  return u;
}

template <typename T>
class Modular {
 public:
  using Type = typename std::decay<decltype(T::value)>::type;

  constexpr static Type mod() { return T::value; }

  constexpr Modular() : value() {}
  template <typename U>
  Modular(const U &x) : value(normalize(x)) {}

  template <typename U>
  Type normalize(const U &x) {
    Type v;
    if (-mod() <= x && x < mod()) {
      v = static_cast<Type>(x);
    } else {
      v = static_cast<Type>(x % mod());
    }
    if (v < 0) v += mod();
    return v;
  }

  const Type &operator()() const { return value; }

  Modular &operator+=(const Modular &rhs) {
    if ((value += rhs.value) >= mod()) value -= mod();
    return *this;
  }
  Modular &operator-=(const Modular &rhs) {
    if ((value -= rhs.value) < 0) value += mod();
    return *this;
  }

  template <typename U>
  Modular &operator+=(const U &rhs) {
    return *this += Modular(rhs);
  }
  template <typename U>
  Modular &operator-=(const U &rhs) {
    return *this -= Modular(rhs);
  }

  Modular &operator++() { return *this += 1; }
  Modular &operator--() { return *this -= 1; }

  Modular operator++(int) {
    Modular res(*this);
    *this += 1;
    return res;
  }
  Modular operator--(int) {
    Modular res(*this);
    *this -= 1;
    return res;
  }

  template <typename U = T>
  typename enable_if<std::is_same<typename Modular<U>::Type, int>::value,
                     Modular>::type &
  operator*=(const Modular &rhs) {
    value = normalize(static_cast<int64_t>(value) *
                      static_cast<int64_t>(rhs.value));
    return *this;
  }
  template <typename U = T>
  typename enable_if<
      std::is_same<typename Modular<U>::Type, int64_t>::value ||
          std::is_same<typename Modular<U>::Type, std::int_fast64_t>::value,
      Modular>::type &
  operator*=(const Modular &rhs) {
    value = normalize(static_cast<__int128>(value) *
                      static_cast<__int128>(rhs.value));
    return *this;
  }

  Modular &operator/=(const Modular &rhs) {
    return *this *= Modular(inverse(rhs.value, mod()));
  }

  template <typename U>
  friend bool operator==(const Modular<U> &lhs, const Modular<U> &rhs);

  template <typename U>
  friend bool operator<(const Modular<U> &lhs, const Modular<U> &rhs);
  template <typename U>
  friend bool operator>(const Modular<U> &lhs, const Modular<U> &rhs);

  template <typename U>
  friend std::istream &operator>>(std::istream &stream, Modular<U> &rhs);
  template <typename U>
  friend std::ostream &operator<<(std::ostream &stream, const Modular<U> &rhs);

 private:
  Type value;
};

template <typename T>
bool operator==(const Modular<T> &lhs, const Modular<T> &rhs) {
  return lhs.value == rhs.value;
}
template <typename T, typename U>
bool operator==(const Modular<T> &lhs, U rhs) {
  return lhs == Modular<T>(rhs);
}
template <typename T, typename U>
bool operator==(U lhs, const Modular<T> &rhs) {
  return Modular<T>(lhs) == rhs;
}

template <typename T>
bool operator!=(const Modular<T> &lhs, const Modular<T> &rhs) {
  return !(lhs == rhs);
}
template <typename T, typename U>
bool operator!=(const Modular<T> &lhs, U rhs) {
  return !(lhs == rhs);
}
template <typename T, typename U>
bool operator!=(U lhs, const Modular<T> &rhs) {
  return !(lhs == rhs);
}

template <typename T>
bool operator<(const Modular<T> &lhs, const Modular<T> &rhs) {
  return lhs.value < rhs.value;
}

template <typename T>
bool operator>(const Modular<T> &lhs, const Modular<T> &rhs) {
  return lhs.value > rhs.value;
}

template <typename T>
Modular<T> operator+(const Modular<T> &lhs, const Modular<T> &rhs) {
  return Modular<T>(lhs) += rhs;
}
template <typename T, typename U>
Modular<T> operator+(const Modular<T> &lhs, U rhs) {
  return Modular<T>(lhs) += rhs;
}
template <typename T, typename U>
Modular<T> operator+(U lhs, const Modular<T> &rhs) {
  return Modular<T>(lhs) += rhs;
}

template <typename T>
Modular<T> operator-(const Modular<T> &lhs, const Modular<T> &rhs) {
  return Modular<T>(lhs) -= rhs;
}
template <typename T, typename U>
Modular<T> operator-(const Modular<T> &lhs, U rhs) {
  return Modular<T>(lhs) -= rhs;
}
template <typename T, typename U>
Modular<T> operator-(U lhs, const Modular<T> &rhs) {
  return Modular<T>(lhs) -= rhs;
}

template <typename T>
Modular<T> operator*(const Modular<T> &lhs, const Modular<T> &rhs) {
  return Modular<T>(lhs) *= rhs;
}
template <typename T, typename U>
Modular<T> operator*(const Modular<T> &lhs, U rhs) {
  return Modular<T>(lhs) *= rhs;
}
template <typename T, typename U>
Modular<T> operator*(U lhs, const Modular<T> &rhs) {
  return Modular<T>(lhs) *= rhs;
}

template <typename T>
Modular<T> operator/(const Modular<T> &lhs, const Modular<T> &rhs) {
  return Modular<T>(lhs) /= rhs;
}
template <typename T, typename U>
Modular<T> operator/(const Modular<T> &lhs, U rhs) {
  return Modular<T>(lhs) /= rhs;
}
template <typename T, typename U>
Modular<T> operator/(U lhs, const Modular<T> &rhs) {
  return Modular<T>(lhs) /= rhs;
}

template <typename T, typename U>
Modular<T> power(const Modular<T> &a, const U &b) {
  assert(b >= 0);
  Modular<T> x = a, res = 1;
  U n = b;
  while (n > 0) {
    if (n & 1) res *= x;
    x *= x;
    n >>= 1;
  }
  return res;
}

template <typename T>
std::istream &operator>>(std::istream &stream, Modular<T> &rhs) {
  typename common_type<typename Modular<T>::Type, int64_t>::type x;
  stream >> x;
  rhs = Modular<T>(x);
  return stream;
}

template <typename T>
std::ostream &operator<<(std::ostream &stream, const Modular<T> &rhs) {
  stream << rhs.value;
  return stream;
}

constexpr int mod = 1000000007;
using mint =
    Modular<std::integral_constant<std::decay<decltype(mod)>::type, mod>>;

template <typename T>
T combination(int n, int r) {
  if (r < 0 || r > n) return 0;
  if (n - r < r) r = n - r;

  T res = 1;
  for (int i = n - r + 1; i < n + 1; i++) res *= i;
  for (int i = 1; i < r + 1; i++) res /= i;
  return res;
}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int N, A, B;
  cin >> N >> A >> B;
  mint res = power(static_cast<mint>(2), N);
  res -= combination<mint>(N, A);
  res -= combination<mint>(N, B);
  cout << res - 1 << '\n';
  return 0;
}