#include <bits/stdc++.h>
using namespace std;
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) begin(v),end(v)
#define fi first
#define se second
template<typename A, typename B> inline bool chmax(A &a, B b) { if (a<b) { a=b; return 1; } return 0; }
template<typename A, typename B> inline bool chmin(A &a, B b) { if (a>b) { a=b; return 1; } return 0; }
using ll = long long;
using pii = pair<int, int>;
constexpr ll INF = 1ll<<30;
constexpr ll longINF = 1ll<<60;
constexpr ll MOD = 1000000007;
constexpr bool debug = 0;
//---------------------------------//

struct UnionFind {
public:
	using size_type = std::size_t;
	
	UnionFind(size_type n) : n(n), size_(n, 1) {
		par.resize(n);
		std::iota(par.begin(), par.end(), 0);
	}
	
	size_type size(size_type x) { return size_[find(x)]; }
	
	size_type find(size_type x) {
		assert(x < n);
		while (par[x] != x) {
			par[x] = par[par[x]];
			x = par[x];
		}
		return x;
	}
	
	void merge(size_type x, size_type y) {
		x = find(x);
		y = find(y);
		if (x == y) return;
		if (size(x) > size(y)) std::swap(x, y);
		par[x] = y;
		size_[y] += size_[x];
	}
	
	bool issame(size_type x, size_type y) { return find(x) == find(y); }
	
private:
	size_type n;
	std::vector<size_type> size_, par;
};

template<int M>
struct ModInt {
public:
	using value_type = long long;
	
	ModInt(value_type val = 0) : val(val < 0 ? (M - (-val % M)) % M : val % M) {}
	
	explicit operator bool() const noexcept { return val; }
	bool operator ==(const ModInt & rhs) const noexcept { return val == rhs.val; }
	bool operator !=(const ModInt & rhs) const noexcept { return !(*this == rhs); }
	ModInt operator +() const noexcept { return ModInt(*this); }
	ModInt operator -() const noexcept { return ModInt(0) -= *this; }
	ModInt operator +(const ModInt & rhs) const noexcept { return ModInt(*this) += rhs; }
	ModInt operator -(const ModInt & rhs) const noexcept { return ModInt(*this) -= rhs; }
	ModInt operator *(const ModInt & rhs) const noexcept { return ModInt(*this) *= rhs; }
	ModInt operator /(const ModInt & rhs) const noexcept { return ModInt(*this) /= rhs; }
	
	ModInt & operator +=(const ModInt & rhs) noexcept {
		val += rhs.val;
		if (val >= M) val -= M;
		return *this;
	}
	ModInt & operator -=(const ModInt & rhs) noexcept {
		if (val < rhs.val) val += M;
		val -= rhs.val;
		return *this;
	}
	ModInt & operator *=(const ModInt & rhs) noexcept {
		val = val * rhs.val % M;
		return *this;
	}
	ModInt & operator /=(const ModInt & rhs) noexcept {
		*this *= rhs.inverse();
		return *this;
	}
	
	ModInt pow(value_type n) const {
		ModInt res = 1, x = val;
		if (n < 0) { x = x.inverse(); n = -n; }
		while (n) { if (n & 1) res *= x; x *= x; n >>= 1; }
		return res;
	}
	
	ModInt inverse() const {
		long long a = val, a1 = 1, a2 = 0, b = M, b1 = 0, b2 = 1;
		while (b > 0) {
			value_type q = a / b, r = a % b;
			value_type nb1 = a1 - q * b1, nb2 = a2 - q * b2;
			a = b; b = r;
			a1 = b1; b1 = nb1;
			a2 = b2; b2 = nb2;
		}
		assert(a == 1);
		return a1;
	}
	
	const value_type & get() const noexcept { return val; }
	static decltype(M) get_mod() noexcept { return M; }
	
	friend std::ostream & operator <<(std::ostream & os, const ModInt & rhs) { return os << rhs.val; }
	friend std::istream & operator >>(std::istream & is, ModInt & rhs) {
		value_type x;
		is >> x;
		rhs = ModInt(x);
		return is;
	}
private:
	value_type val;
};
using mint = ModInt<MOD>;

template<typename T>
struct Combination {
public:
	using size_type = std::size_t;
	
	Combination(size_type sz = 1) : _fact(1, 1), _finv(1, 1), _inv(1, 1) { build(sz); }
	
	T fact(size_type k) { if (k >= T::get_mod()) return 0; build(k); return _fact[k]; }
	T finv(size_type k) { assert(k < T::get_mod()); build(k); return _finv[k]; }
	T inv(size_type k) { assert(k > 0 && k < T::get_mod()); build(k); return _inv[k]; }
	
	T operator ()(int n, int r) { return c(n, r); }
	T c(int n, int r) {
		if (r < 0 || n < r) return 0;
		return fact(n) * finv(r) * finv(n - r);
	}
	
private:
	std::vector<T> _fact, _finv, _inv;
	static constexpr size_type MAX_LIMIT = 50000000;
	
	void build(size_type k) {
		if (_fact.size() > k) return;
		assert(k < MAX_LIMIT);
		size_type sz = std::min({MAX_LIMIT, static_cast<size_type>(T::get_mod()), std::max(_fact.size() * 2, k + 1)});
		size_type presz = _fact.size();
		_fact.resize(sz);
		_finv.resize(sz);
		_inv.resize(sz);
		
		for (size_type i = presz; i < sz; ++i) _fact[i] = _fact[i - 1] * i;
		_finv[sz - 1] = T(_fact[sz - 1]).inverse();
		for (size_type i = sz - 1; i > presz; --i) {
			_finv[i - 1] = _finv[i] * i;
			_inv[i] = _fact[i - 1] * _finv[i];
		}
		_inv[presz] = _fact[presz - 1] * _finv[presz];
	}
};
using Comb = Combination<mint>;
Comb comb;

int main() {
	int N;
	cin >> N;
	vector<int> P(N);
	REP(i, N) {
		scanf("%d", &P[i]);
		if (P[i] > 0) --P[i];
	}
	
	UnionFind uf(N);
	vector<int> bloop, m1;
	REP(i, N) {
		if (P[i] == -1) m1.emplace_back(i);
		else if (uf.issame(i, P[i])) bloop.emplace_back(i);
		else uf.merge(i, P[i]);
	}
	
	mint ans = mint(N - 1).pow(m1.size()) * bloop.size();
	vector<vector<mint>> dp(m1.size() + 1, vector<mint>(m1.size() + 1));
	dp[0][0] = 1;
	REP(i, m1.size()) {
		REP(j, m1.size()) {
			if (dp[i][j] == 0) continue;
			dp[i + 1][j] += dp[i][j];
			dp[i + 1][j + 1] += dp[i][j] * uf.size(m1[i]);
		}
		ans += mint(N - 1).pow(m1.size() - 1) * (uf.size(m1[i]) - 1);
	}
	
	FOR(i, 2, m1.size() + 1) ans += dp[m1.size()][i] * comb.fact(i - 1) * mint(N - 1).pow(m1.size() - i);
	
	ans = mint(N - 1).pow(m1.size()) * N - ans;
	cout << ans << endl;
	
	return 0;
}


Measured memory_bytes: 199112