#include <bits/stdc++.h>
using namespace std;

using ll = long long;
using Pi = pair<int, int>;
using Pl = pair<ll, ll>;
using vint = vector<int>;
using vvint = vector<vint>;
using vvvint = vector<vvint>;
using vdouble = vector<double>;
using vvdouble = vector<vdouble>;
using vvvdouble = vector<vvdouble>;
using vll = vector<ll>;
using vvll = vector<vll>;
using vvvll = vector<vvll>;
using uint = unsigned int;
using ull = unsigned long long;

template<typename T> using uset = unordered_set<T>;
template<typename T1, typename T2> using umap = unordered_map<T1, T2>;

constexpr int INF = (1 << 30) - 1;
constexpr ll LLINF = 1LL << 60;
constexpr int dy[] = {1, 0, -1, 0, 1, -1, -1, 1};
constexpr int dx[] = {0, 1, 0, -1, 1, 1, -1, -1};
constexpr char el = '\n';
constexpr int mod = 1000000007;
constexpr int mod2 = 998244353;
template<typename T> T gcd(T a, T b) { return (b ? gcd(b, a % b) : a); }
template<typename T> T lcm(T a, T b) { return (a / gcd(a, b) * b); }
template<typename T1, typename T2>
inline bool chmin(T1 &a, T2 b) { return (a > b && (a = b, true)); }
template<typename T1, typename T2>
inline bool chmax(T1 &a, T2 b) { return (a < b && (a = b, true)); }
template<typename T>
bool isin(T y, T x, T h, T w) { return (0 <= y && 0 <= x && y < h && x < w); }
template<typename T>
bool isin1(T y, T x, T h, T w) { return (0 < y && 0 < x && y <= h && x <= w); }

template<typename T>
ostream& operator <<(ostream &os, vector<T> &v) {
	os << v[0];
	for (int i = 1; i < v.size(); i++) os << " " << v[i];
	return (os);
}

template<typename T>
istream& operator >>(istream &is, vector<T> &v) {
	for (auto &u : v) is >> u;
	return (is);
}

template<typename T1, typename T2>
istream& operator >>(istream &is, pair<T1, T2> &p) {
	return (is >> p.first >> p.second);
}

vvint G;
bitset<5005> flag;
vint P;

Pi coloring(int u) {
	Pi ret = Pi(1, 0);
	if (P[u] == -1) ret.second = true;

	if (flag[u]) return (Pi(0, 0));
	flag[u] = true;

	for (auto &v : G[u]) {
		Pi val = coloring(v);
		ret.first += val.first;
		ret.second |= val.second;
	}
	return (ret);
}

void Main() {
	int N; cin >> N;
	G = vvint(N);
	P = vint(N); cin >> P;
	for (int i = 0; i < N; i++) {
		if (P[i] > 0) --P[i];
		if (P[i] == -1) continue;
		G[i].push_back(P[i]);
		G[P[i]].push_back(i);
	}

	vll powNd(N+1, 1);
	for (int i = 1; i <= N; i++) {
		(powNd[i] *= (powNd[i-1] * (N-1)) % mod) %= mod;
	}

	vll S;
	int n = 0;
	ll sum = 0;
	ll betsu = 0;
	for (int i = 0; i < N; i++) {
		if (!flag[i]) {
			Pi val = coloring(i);
			if (val.second == 0) {
				betsu++;
				continue;
			}
			S.push_back(val.first);
			n++;
			sum += S.back()-1;
		}
	}

	vvll dp(n+1, vll(n+1, 0));
	dp[0][0] = 1;
	for (int i = 0; i <= n; i++) {
		for (int j = 0; j <= n; j++) {
			if (i < n && j < n) (dp[i+1][j+1] += dp[i][j] * S[j]) %= mod;
			if (j < n) (dp[i][j+1] += dp[i][j]) %= mod;
		}
	}
	
	
	(sum *= powNd[n-1]) %= mod;
	ll fact = 1;
	for (int i = 2; i <= n; i++) {
		(fact *= (i-1)) %= mod;
		(sum += ((dp[i][n] * fact) % mod * powNd[n-i]) % mod) %= mod;
	}

	cout << ((N-betsu) * powNd[n] + mod - sum) % mod << endl;
}

int main() {
	cin.tie(0);
	ios::sync_with_stdio(0);
	cout << fixed << setprecision(20);
	Main();
	return (0);
}


Measured memory_bytes: 198936