// Author: Tyler Wang

#include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <cctype>
#include <chrono>
#include <cmath>
#include <complex>
#include <cstdint>
#include <cstdlib>
#include <deque>
#include <functional>
#include <initializer_list>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <limits>
#include <map>
#include <memory>
#include <numeric>
#include <optional>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <string_view>
#include <tuple>
#include <type_traits>
#include <utility>
#include <vector>

using namespace std;

template <class C>
constexpr auto ssize(const C& c)
    -> std::common_type_t<std::ptrdiff_t,
                          std::make_signed_t<decltype(c.size())>> {
  return (std::common_type_t<std::ptrdiff_t,
                             std::make_signed_t<decltype(c.size())>>)c.size();
}

template <class T, std::ptrdiff_t N>
constexpr std::ptrdiff_t ssize(const T (&)[N]) noexcept {
  return N;
}

template <typename T>
constexpr std::enable_if_t<std::is_unsigned_v<T>, bool> has_single_bit(
    T x) noexcept {
  return x != 0 && (x & (x - 1)) == 0;
}

template <typename T>
constexpr std::enable_if_t<std::is_unsigned_v<T>, int> countl_zero(
    T x) noexcept {
  if constexpr (sizeof(T) <= sizeof(unsigned)) {
    return __builtin_clz(x) - (std::numeric_limits<unsigned>::digits -
                               std::numeric_limits<T>::digits);
  } else if constexpr (sizeof(T) <= sizeof(unsigned long)) {
    return __builtin_clzl(x) - (std::numeric_limits<unsigned long>::digits -
                                std::numeric_limits<T>::digits);
  } else {
    static_assert(sizeof(T) <= sizeof(unsigned long long));
    return __builtin_clzll(x) -
           (std::numeric_limits<unsigned long long>::digits -
            std::numeric_limits<T>::digits);
  }
}

template <typename T>
constexpr std::enable_if_t<std::is_unsigned_v<T>, int> countr_zero(
    T x) noexcept {
  if constexpr (sizeof(T) <= sizeof(unsigned)) {
    return __builtin_ctz(x);
  } else if constexpr (sizeof(T) <= sizeof(unsigned long)) {
    return __builtin_ctzl(x);
  } else {
    static_assert(sizeof(T) <= sizeof(unsigned long long));
    return __builtin_ctzll(x);
  }
}

template <typename T>
constexpr std::enable_if_t<std::is_unsigned_v<T>, int> popcount(T x) noexcept {
  if constexpr (sizeof(T) <= sizeof(unsigned)) {
    return __builtin_popcount(x);
  } else if constexpr (sizeof(T) <= sizeof(unsigned long long)) {
    return __builtin_popcountl(x);
  } else {
    static_assert(sizeof(T) <= sizeof(unsigned long long));
    return __builtin_popcountll(x);
  }
}

template <typename T>
constexpr std::enable_if_t<std::is_unsigned_v<T>, T> bit_width(T x) noexcept {
  return (T)(std::numeric_limits<T>::digits - countl_zero(x));
}

template <typename T>
constexpr std::enable_if_t<std::is_unsigned_v<T>, T> bit_ceil(T x) noexcept {
  return (T)(x <= 1 ? 1 : (T)1 << bit_width((T)(x - 1)));
}

template <typename T>
constexpr std::enable_if_t<std::is_unsigned_v<T>, T> bit_floor(T x) noexcept {
  return (T)(x == 0 ? 0 : (T)1 << (bit_width(x) - 1));
}

template <class Fun>
class y_combinator_result {
 public:
  template <class T>
  explicit y_combinator_result(T&& fun) : fun_(std::forward<T>(fun)) {}

  template <class... Args>
  decltype(auto) operator()(Args&&... args) {
    return fun_(std::ref(*this), std::forward<Args>(args)...);
  }

 private:
  Fun fun_;
};

template <class Fun>
decltype(auto) y_combinator(Fun&& fun) {
  return y_combinator_result<std::decay_t<Fun>>(std::forward<Fun>(fun));
}

template <typename T>
std::ostream& operator<<(std::ostream& os, const std::optional<T>& opt) {
  return opt ? os << *opt : os << "nullopt";
}

template <typename T1, typename T2>
std::ostream& operator<<(std::ostream& os, const std::pair<T1, T2>& p) {
  return os << '{' << p.first << ", " << p.second << '}';
}

template <typename Container,
          std::enable_if_t<!std::is_convertible_v<Container, std::string_view>,
                           typename Container::const_iterator>* = nullptr>
std::ostream& operator<<(std::ostream& os, const Container& c) {
  os << '{';
  for (auto it = c.begin(); it != c.end(); ++it) {
    if (it != c.begin()) {
      os << ", ";
    }
    os << *it;
  }
  return os << '}';
}

template <typename T, typename Container>
std::ostream& operator<<(std::ostream& os, std::stack<T, Container> s) {
  std::vector<T> v(s.size());
  for (auto it = v.rbegin(); it != v.rend(); ++it) {
    *it = s.top();
    s.pop();
  }
  return os << v;
}

template <typename T, typename Container>
std::ostream& operator<<(std::ostream& os, std::queue<T, Container> q) {
  os << '{';
  if (!q.empty()) {
    os << q.front();
    q.pop();
    while (!q.empty()) {
      os << ", " << q.front();
      q.pop();
    }
  }
  return os << '}';
}

template <typename T, typename Container, typename Compare>
std::ostream& operator<<(std::ostream& os,
                         std::priority_queue<T, Container, Compare> pq) {
  os << '{';
  if (!pq.empty()) {
    os << pq.top();
    pq.pop();
    while (!pq.empty()) {
      os << ", " << pq.top();
      pq.pop();
    }
  }
  return os << '}';
}

namespace detail {

template <int I, typename... Ts>
void tuple_print(std::ostream& os, const std::tuple<Ts...>& t) {
  static_assert(0 <= I && I <= sizeof...(Ts));
  if constexpr (I != sizeof...(Ts)) {
    if constexpr (I != 0) {
      os << ", ";
    }
    os << std::get<I>(t);
    tuple_print<I + 1>(os, t);
  }
}

}  // namespace detail

template <typename... Ts>
std::ostream& operator<<(std::ostream& os, const std::tuple<Ts...>& t) {
  os << '{';
  detail::tuple_print<0>(os, t);
  return os << '}';
}

namespace detail {

template <typename T1, typename... Ts>
void debug_print(std::string_view keys, const T1& val1, const Ts&... vals) {
  if constexpr (sizeof...(Ts) == 0) {
    std::cerr << keys << " = " << val1 << std::endl;
  } else {
    for (int i = 0, paren = 0; i < (int)keys.size(); ++i) {
      if (keys[i] == '(' || keys[i] == '{') {
        ++paren;
      } else if (keys[i] == ')' || keys[i] == '}') {
        --paren;
      } else if (keys[i] == ',' && paren == 0) {
        std::cerr << keys.substr(0, i) << " = " << val1 << ',';
        return debug_print(keys.substr(i + 1), vals...);
      }
    }
  }
}

template <typename... Ts>
void debug_print(int line, const Ts&... keyvals) {
  if constexpr (sizeof...(Ts) <= 1) {
    std::cerr << "  " << line << " |" << std::endl;
  } else {
    std::cerr << "  " << line << " | ";
    debug_print(keyvals...);
  }
}

}  // namespace detail

#if defined(TYLER) && defined(__GNUC__) && __GNUC__ >= 8
#define debug(...) \
  detail::debug_print(__LINE__, #__VA_ARGS__ __VA_OPT__(, ) __VA_ARGS__)
#elif defined(TYLER)
#define debug(...) detail::debug_print(__LINE__, #__VA_ARGS__, __VA_ARGS__)
#else
#define debug(...) ((void)0)
#endif

class disjoint_sets {
 public:
  disjoint_sets(int n) : count_(n), parent_or_size_(n, -1) {}

  int count() const { return count_; }

  int size() const { return (int)parent_or_size_.size(); }

  int size(int x) { return -parent_or_size_[find(x)]; }

  int find(int x) {
    if (parent_or_size_[x] < 0) {
      return x;
    }
    return parent_or_size_[x] = find(parent_or_size_[x]);
  }

  bool join(int x, int y) {
    if ((x = find(x)) == (y = find(y))) {
      return false;
    }
    if (parent_or_size_[x] > parent_or_size_[y]) {
      std::swap(x, y);
    }
    parent_or_size_[x] += parent_or_size_[y];
    parent_or_size_[y] = x;
    --count_;
    return true;
  }

  std::vector<std::vector<int>> sets() {
    std::vector<int> order(size(), -1);
    std::vector<std::vector<int>> subsets(count());
    for (int i = 0, k = 0; i < size(); ++i) {
      int p = find(i);
      if (order[p] == -1) {
        order[p] = k++;
      }
      subsets[order[p]].push_back(i);
    }
    return subsets;
  }

  friend std::ostream& operator<<(std::ostream& os, disjoint_sets ds) {
    return os << ds.sets();
  }

 private:
  int count_;
  std::vector<int> parent_or_size_;
};

constexpr int MODULUS = 1'000'000'007;

template <const int& Modulus = MODULUS>
class mod_int {
 public:
  constexpr mod_int() : val_(0) {}

  template <typename T>
  constexpr mod_int(T val) : val_((int)(val % Modulus)) {
    static_assert(std::is_integral_v<T>);
    if (val_ < 0) {
      val_ += Modulus;
    }
  }

  static constexpr int modulus() { return Modulus; }

  template <typename T>
  constexpr explicit operator T() const {
    return (T)val_;
  }

  constexpr mod_int& operator++() {
    if (++val_ == Modulus) {
      val_ = 0;
    }
    return *this;
  }

  constexpr mod_int operator++(int) {
    mod_int t = *this;
    ++*this;
    return t;
  }

  constexpr mod_int& operator--() {
    if (val_ == 0) {
      val_ = Modulus;
    }
    --val_;
    return *this;
  }

  constexpr mod_int operator--(int) {
    mod_int t = *this;
    --*this;
    return t;
  }

  constexpr mod_int operator+=(mod_int other) {
    if ((val_ += other.val_) >= Modulus) {
      val_ -= Modulus;
    }
    return *this;
  }

  constexpr mod_int operator-=(mod_int other) {
    if ((val_ -= other.val_) < 0) {
      val_ += Modulus;
    }
    return *this;
  }

  constexpr mod_int operator*=(mod_int other) {
    val_ = (int)((long long)val_ * other.val_ % Modulus);
    return *this;
  }

  constexpr mod_int operator/=(mod_int other) { return *this *= inv(other); }

  friend constexpr mod_int operator+(mod_int mi) { return mi; }

  friend constexpr mod_int operator-(mod_int mi) {
    if (mi.val_ != 0) {
      mi.val_ = Modulus - mi.val_;
    }
    return mi;
  }

  friend constexpr mod_int operator+(mod_int lhs, mod_int rhs) {
    return lhs += rhs;
  }

  friend constexpr mod_int operator-(mod_int lhs, mod_int rhs) {
    return lhs -= rhs;
  }

  friend constexpr mod_int operator*(mod_int lhs, mod_int rhs) {
    return lhs *= rhs;
  }

  friend constexpr mod_int operator/(mod_int lhs, mod_int rhs) {
    return lhs /= rhs;
  }

  friend constexpr bool operator==(mod_int lhs, mod_int rhs) {
    return lhs.val_ == rhs.val_;
  }

  friend constexpr bool operator!=(mod_int lhs, mod_int rhs) {
    return lhs.val_ != rhs.val_;
  }

  friend constexpr bool operator<(mod_int lhs, mod_int rhs) {
    return lhs.val_ < rhs.val_;
  }

  friend constexpr bool operator<=(mod_int lhs, mod_int rhs) {
    return lhs.val_ <= rhs.val_;
  }

  friend constexpr bool operator>(mod_int lhs, mod_int rhs) {
    return lhs.val_ > rhs.val_;
  }

  friend constexpr bool operator>=(mod_int lhs, mod_int rhs) {
    return lhs.val_ >= rhs.val_;
  }

  friend std::ostream& operator<<(std::ostream& os, mod_int mi) {
    return os << mi.val_;
  }

  friend std::istream& operator>>(std::istream& is, mod_int& mi) {
    long long val;
    is >> val;
    mi = mod_int(val);
    return is;
  }

  friend constexpr mod_int inv(mod_int mi) {
    int a = mi.val_, b = Modulus;
    int x = 0, xx = 1;
    while (a != 0) {
      int q = b / a;
      int t = a;
      a = b - q * a;
      b = t;
      t = xx;
      xx = x - q * xx;
      x = t;
    }
    return mod_int(x);
  }

  template <typename T>
  friend constexpr mod_int pow(mod_int base, T expo) {
    static_assert(std::is_integral_v<T>);
    if (expo < 0) {
      base = inv(base);
      expo = -expo;
    }
    mod_int power = 1;
    for (; expo != 0; expo >>= 1) {
      if (expo & 1) {
        power *= base;
      }
      base *= base;
    }
    return power;
  }

 private:
  int val_;
};

// Modular combinatorics.
template <const int& Modulus = MODULUS>
class mod_comb {
 public:
  mod_comb(int n) : fact_(n + 1), inv_fact_(n + 1) {
    fact_[0] = 1;
    for (int i = 1; i <= n; ++i) {
      fact_[i] = fact_[i - 1] * i;
    }
    inv_fact_[n] = inv(fact_[n]);
    for (int i = n; i >= 1; --i) {
      inv_fact_[i - 1] = inv_fact_[i] * i;
    }
  }

  mod_int<Modulus> factorial(int n) const { return fact_[n]; }

  mod_int<Modulus> permute(int n, int k) const {
    return fact_[n] * inv_fact_[n - k];
  }

  mod_int<Modulus> choose(int n, int k) const {
    return fact_[n] * inv_fact_[n - k] * inv_fact_[k];
  }

 private:
  std::vector<mod_int<Modulus>> fact_, inv_fact_;
};

int main() {
  std::ios::sync_with_stdio(false);
  std::cin.tie(nullptr);
  std::cout.precision(12);
  std::cerr.precision(12);
  std::mt19937 rng((std::uint_fast32_t)std::chrono::high_resolution_clock::now()
                       .time_since_epoch()
                       .count());
  int n;
  cin >> n;
  disjoint_sets ds(n);
  vector<int> p(n), unknown;
  for (int i = 0; i < n; ++i) {
    cin >> p[i];
    if (p[i] == -1) {
      unknown.push_back(i);
    } else {
      --p[i];
      ds.join(i, p[i]);
    }
  }
  int m = (int)unknown.size();
  vector<vector<mod_int<>>> dp(m + 1, vector<mod_int<>>(m + 1));
  dp[0][0] = 1;
  for (int i = 0; i < m; ++i) {
    for (int j = 0; j <= i; ++j) {
      dp[i + 1][j] += dp[i][j];
      dp[i + 1][j + 1] += dp[i][j] * ds.size(unknown[i]);
    }
  }
  mod_comb comb(m);
  mod_int ans = n * pow(mod_int(n - 1), m);
  ans -= (ds.count() - m) * pow(mod_int(n - 1), m);
  dp[m][1] -= m;
  for (int j = 1; j <= m; ++j) {
    ans -= dp[m][j] * comb.factorial(j - 1) * pow(mod_int(n - 1), m - j);
  }
  cout << ans << '\n';
  return 0;
}

Measured memory_bytes: 101140