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// need
#include <iostream>
#include <algorithm>
// data structure
#include <bitset>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <utility>
#include <vector>
#include <complex>
//#include <deque>
#include <valarray>
#include <unordered_map>
#include <unordered_set>
#include <array>
// etc
#include <cassert>
#include <cmath>
#include <functional>
#include <iomanip>
#include <chrono>
#include <random>
#include <numeric>
#include <fstream>

// input
#define INIT std::ios::sync_with_stdio(false);std::cin.tie(0);
#define VAR(type, ...)type __VA_ARGS__;MACRO_VAR_Scan(__VA_ARGS__);
template<typename T> void MACRO_VAR_Scan(T& t) { std::cin >> t; }
template<typename First, typename...Rest>void MACRO_VAR_Scan(First& first, Rest& ...rest) { std::cin >> first; MACRO_VAR_Scan(rest...); }
#define VEC_ROW(type, n, ...)std::vector<type> __VA_ARGS__;MACRO_VEC_ROW_Init(n, __VA_ARGS__); for(int w_=0; w_<n; ++w_){MACRO_VEC_ROW_Scan(w_, __VA_ARGS__);}
template<typename T> void MACRO_VEC_ROW_Init(int n, T& t) { t.resize(n); }
template<typename First, typename...Rest>void MACRO_VEC_ROW_Init(int n, First& first, Rest& ...rest) { first.resize(n); MACRO_VEC_ROW_Init(n, rest...); }
template<typename T> void MACRO_VEC_ROW_Scan(int p, T& t) { std::cin >> t[p]; }
template<typename First, typename...Rest>void MACRO_VEC_ROW_Scan(int p, First& first, Rest& ...rest) { std::cin >> first[p]; MACRO_VEC_ROW_Scan(p, rest...); }
#define VEC(type, c, n) std::vector<type> c(n);for(auto& i:c)std::cin>>i;
#define MAT(type, c, m, n) std::vector<std::vector<type>> c(m, std::vector<type>(n));for(auto& R:c)for(auto& w:R)std::cin>>w;
// output
template<typename T>void MACRO_OUT(const T t) { std::cout << t; }
template<typename First, typename...Rest>void MACRO_OUT(const First first, const Rest...rest) { std::cout << first << " "; MACRO_OUT(rest...); }
#define OUT(...) MACRO_OUT(__VA_ARGS__);
#define FOUT(n, dist) std::cout<<std::fixed<<std::setprecision(n)<<(dist);
#define SOUT(n, c, dist) std::cout<<std::setw(n)<<std::setfill(c)<<(dist);
#define EOUT(...) { OUT(__VA_ARGS__)BR; exit(0); }
#define SP std::cout<<" ";
#define TAB std::cout<<"\t";
#define BR std::cout<<"\n";
#define SPBR(w, n) std::cout<<(w + 1 == n ? '\n' : ' ');
#define ENDL std::cout<<std::endl;
#define FLUSH std::cout<<std::flush;
#define SHOW(dist) {std::cerr << #dist << "\t: " << (dist) << "\n";}
#define SHOWVECTOR(v) {std::cerr << #v << "\t: ";for(const auto& xxx : v){std::cerr << xxx << " ";}std::cerr << "\n";}
#define SHOWVECTOR2(v) {std::cerr << #v << "\t:\n";for(const auto& xxx : v){for(const auto& yyy : xxx){std::cerr << yyy << " ";}std::cerr << "\n";}}
#define SHOWQUEUE(a) {auto tmp(a);std::cerr << #a << "\t: ";while(!tmp.empty()){std::cerr << tmp.front() << " ";tmp.pop();}std::cerr << "\n";}
#define SHOWSTACK(a) {auto tmp(a);std::cerr << #a << "\t: ";while(!tmp.empty()){std::cerr << tmp.top() << " ";tmp.pop();}std::cerr << "\n";}
// utility
#define ALL(a) (a).begin(),(a).end()
#define FOR(w, a, n) for(int w=(a);w<(n);++w)
#define REP(w, n) FOR(w, 0, n)
#define RFOR(w, a, n) for(int w=(n)-1;w>=(a);--w)
#define RREP(w, n) RFOR(w, 0, n)
template<class S, class T, class U> bool IN(S a, T x, U b) { return a <= x && x < b; }
template<class T> inline T CHMAX(T& a, const T b) { return a = (a < b) ? b : a; }
template<class T> inline T CHMIN(T& a, const T b) { return a = (a > b) ? b : a; }
// test
template<class T> using V = std::vector<T>;
template<class T> using VV = V<V<T>>;

template<typename S, typename T>
std::ostream& operator<<(std::ostream& os, std::pair<S, T> p) {
	os << "(" << p.first << ", " << p.second << ")"; return os;
}

// type/const
using i64 = std::int_fast64_t;
using u64 = std::uint_fast64_t;
using ll = long long;
using ull = unsigned long long;
using ld = long double;
using PAIR = std::pair<i64, i64>;
using PAIRLL = std::pair<ll, ll>;
constexpr int INFINT = (1 << 30) - 1;                    // 1.07x10^ 9
constexpr int INFINT_LIM = (1LL << 31) - 1;              // 2.15x10^ 9
constexpr ll INFLL = 1LL << 60;                          // 1.15x10^18
constexpr ll INFLL_LIM = (1LL << 62) - 1 + (1LL << 62);  // 9.22x10^18
constexpr double EPS = 1e-6;
constexpr i64 MOD = 1000000007;
constexpr double PI = 3.141592653589793238462643383279;
template<class T, size_t N> void FILL(T(&a)[N], const T & val) { for (auto& x : a) x = val; }
template<class ARY, size_t N, size_t M, class T> void FILL(ARY(&a)[N][M], const T & val) { for (auto& b : a) FILL(b, val); }
template<class T> void FILL(std::vector<T> & a, const T & val) { for (auto& x : a) x = val; }
template<class ARY, class T> void FILL(std::vector<std::vector<ARY>> & a, const T & val) { for (auto& b : a) FILL(b, val); }
// ------------>8---------

ll powMod(ll n, ll p) {
	ll res = 1;
	while (p) {
		if (p & 1) (res *= n) %= MOD;
		(n *= n) %= MOD;
		p >>= 1;
	}
	return res;
}
std::map<int, int> inv;
ll invMod(ll n) {
	if (inv.count(n)) return inv[n];
	return inv[n] = powMod(n, MOD - 2);
}

class UnionFind {
private:
	std::vector<int> par;
	std::vector<int> siz;

public:
	UnionFind(int sz_) : par(sz_), siz(sz_, 1) {
		for (int i = 0; i < sz_; ++i) par[i] = i;
	}
	void init(int sz_) {
		par.resize(sz_);
		siz.resize(sz_, 1);
		for (int i = 0; i < sz_; ++i) par[i] = i;
	}
	int find(int x) {
		while (par[x] != x) x = par[x] = par[par[x]];
		return x;
	}
	void unite(int x, int y) {
		x = find(x);
		y = find(y);
		if (x == y) return;
		if (siz[x] < siz[y]) std::swap(x, y);
		siz[x] += siz[y];
		par[y] = x;
	}
	bool same(int x, int y) {
		return find(x) == find(y);
	}
	int size(int x) {
		return siz[find(x)];
	}
};

i64 dp[5003][5003];

signed main() {
	INIT;

	VAR(int, n);
	VEC(int, p, n);
	REP(i, n) --p[i];

	V<PAIR> a;
	{
		UnionFind uf(n);
		REP(i, n) if (p[i] >= 0) uf.unite(i, p[i]);
		V<int> cnt(n, 0);
		REP(i, n) if (p[i] < 0) ++cnt[uf.find(i)];
		REP(i, n) {
			if (uf.find(i) != i) continue;
			a.emplace_back(uf.size(i), cnt[i]);
		}
	}
	int sz = a.size();

	V<int> c(sz + 1, 0);
	REP(i, sz) c[i + 1] = c[i] + a[i].first;

	dp[0][0] = 1;
	REP(i, sz) REP(j, i + 1) {
		if (dp[i][j] == 0) continue;
		// add
		if (a[i].second) {
			(dp[i + 1][j + 1] += dp[i][j] * a[i].first) %= MOD;
		}

		// ignore
		if (a[i].second) {
			(dp[i + 1][j] += dp[i][j] * (n - 1)) %= MOD;
		}
		else {
			(dp[i + 1][j] += dp[i][j]) %= MOD;
		}
	}

	i64 ans = n;
	i64 all = 1;
	REP(i, n) if (p[i] < 0) (all *= n - 1) %= MOD;
	(ans *= all) %= MOD;

	// 0
	REP(i, sz) if (a[i].second == 0) (ans -= all) %= MOD;

	// 1
	REP(i, sz) if (a[i].second) (ans -= (a[i].first - 1) * all % MOD * invMod(n - 1)) %= MOD;

	// 2~
	i64 t = 1;
	FOR(i, 2, sz + 1) {
		(ans -= dp[sz][i] * t) %= MOD;
		(t *= i) %= MOD;
	}

	if (ans < 0) ans += MOD;
	OUT(ans)BR;

	return 0;
}

Measured memory_bytes: 120480