#include <bits/stdc++.h>

using namespace std;

//using ll = int64_t;
using ll = long long;
using ull = uint64_t;
using i32 = int32_t;
using u32 = uint32_t;
using i64 = int64_t;
using u64 = uint64_t;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using ld = double;

#define X first
#define Y second

#ifndef ONLINE_JUDGE
#define FWRITE
#endif

namespace io
{

#ifndef FWRITE
#include <unistd.h>
	auto unistd_read = read;
	auto unistd_write = write;
#endif
	const int BUFSIZE = 1 << 20;

	int isize, osize;
	char ibuf[BUFSIZE + 10], obuf[BUFSIZE + 10];
	char* is, * it, * os = obuf, * ot = obuf + BUFSIZE;

	char getchar()
	{
		if (is == it)
		{
			is = ibuf;
#ifdef FWRITE
			it = ibuf + fread(ibuf, 1, BUFSIZE, stdin);
#else
			it = ibuf + unistd_read(STDIN_FILENO, ibuf, BUFSIZE);
#endif
			if (is == it) return EOF;
		}
		return *is++;
	}

	char getalpha()
	{
		char c = getchar();
		while (!isalpha(c)) c = getchar();
		return c;
	}

	void putchar(char c)
	{
		*os++ = c;
		if (os == ot)
		{
#ifdef FWRITE
			fwrite(obuf, 1, BUFSIZE, stdout);
#else
			unistd_write(STDOUT_FILENO, obuf, BUFSIZE);
#endif
			os = obuf;
		}
	}

	int inp() {
		int x = 0, f = 0; char ch;
		for (ch = getchar(); !isdigit(ch); ch = getchar())
		{
			if (ch == EOF) return -1;
			if (ch == '-') f = 1;
		}
		for (; isdigit(ch); x = x * 10 + ch - '0', ch = getchar());
		return f ? -x : x;
	}

	ll inp_ll() {
		ll x = 0; int f = 0; char ch;
		for (ch = getchar(); !isdigit(ch); ch = getchar())
			if (ch == '-') f = 1;
		for (; isdigit(ch); x = x * 10 + ch - '0', ch = getchar());
		return f ? -x : x;
	}

	template<class T>
	bool read(T& x)
	{
		x = 0;
		int f = 0; char ch;
		for (ch = getchar(); !isdigit(ch); ch = getchar())
		{
			if (ch == EOF) return 0;
			if (ch == '-') f = 1;
		}
		for (; isdigit(ch); x = x * 10 + ch - '0', ch = getchar());
		if (f) x = -x;
		return 1;
	}

	bool read(char* s)
	{
		char* t = s;
		char ch = getchar();
		for (; ch == ' ' || ch == '\n'; ch = getchar());
		for (; ch != ' ' && ch != '\n' && ch != EOF; ch = getchar())
			*t++ = ch;
		*t = 0;
		return s != t;
	}

	template<class T, class... Args>
	bool read(T& x, Args&... args)
	{
		return read(x) && read(args...);
	}

	template<class T>
	bool readln(T& x)
	{
		x = 0;
		int f = 0; char ch;
		for (ch = getchar(); !isdigit(ch); ch = getchar())
		{
			if (ch == EOF) return 0;
			if (ch == '-') f = 1;
		}
		for (; isdigit(ch); x = x * 10 + ch - '0', ch = getchar());
		if (f) x = -x;
		for (; ch != '\n' && ch != EOF; ch = getchar());
		return 1;
	}

	bool readln(char* s)
	{
		char* t = s;
		while (1)
		{
			char ch = getchar();
			if (ch == '\n' || ch == EOF) break;
			*t++ = ch;
		}
		*t = 0;
		return s != t;
	}

	template<class T, class... Args>
	bool readln(T& x, Args&... args)
	{
		return read(x) && readln(args...);
	}

	template<class T>
	void write(T x)
	{
		static char s[22];
		static char* it = s + 20;
		static char* end = s + 20;

		if (x < 0)
		{
			putchar('-');
			x = -x;
		}

		do
		{
			*--it = x % 10 + '0';
			x /= 10;
		} while (x);
		/*
		if (!x)
			*-- it = '0';
		while (x)
		{
			*-- it = x%10+'0';
			x /= 10;
		}
		*/
		for (; it < end; ++it)
			putchar(*it);
	}

	void write(const char* s)
	{
		for (; *s; ++s) putchar(*s);
	}

	template<>
	void write(char* s)
	{
		write((const char*)s);
	}

	template<>
	void write(char c)
	{
		putchar(c);
	}

	template<class T, class... Args>
	void write(T x, Args... args)
	{
		write(x);
		write(args...);
	}

	void writeln()
	{
		putchar('\n');
	}

	template<class T, class... Args>
	void writeln(T x, Args... args)
	{
		write(x);
		writeln(args...);
	}

	template<class Iterator>
	bool input(Iterator st, Iterator ed)
	{
		for (; st != ed; ++st)
		{
			if (!read(*st)) return false;
		}
		return true;
	}

	template<class T>
	bool input(T& a)
	{
		input(a.begin(), a.end());
	}

	template<class Iterator>
	void print(Iterator st, Iterator ed, const char* c = " ")
	{
		int flag = 0;
		for (; st != ed; ++st)
		{
			if (flag) write(c);
			flag = 1;
			write(*st);
		}
		writeln();
	}

	template<class T>
	void print(const T& a, const char* c = " ")
	{
		print(a.begin(), a.end(), c);
	}

	struct ender
	{
		~ender()
		{
			if (os != obuf)
#ifdef FWRITE
				fwrite(obuf, 1, os - obuf, stdout);
#else
				unistd_write(STDOUT_FILENO, obuf, os - obuf);
#endif
		}
	}__ender;

}

int64_t power(int64_t a, int64_t b, int64_t p)
{
	if (!b) return 1;
	int64_t t = power(a, b >> 1, p);
	t = t * t % p;
	if (b & 1) t = t * a % p;
	return t;
}

//mt19937_64 rd(chrono::steady_clock::now().time_since_epoch().count());
mt19937 rd(chrono::steady_clock::now().time_since_epoch().count());

using namespace io;

template<class T>
inline void freshmin(T& a, const T& b)
{
	if (a > b) a = b;
}

template<class T>
inline void freshmax(T& a, const T& b)
{
	if (a < b) a = b;
}

const ll B = 31;
//const ll MOD = 1000000007;
const int INF = 1000000010;
const ll INFll = 4000000000000000000LL;
const int MAXN = 5010;

//int dx[] = { -1, 1, 0, 0, -1, -1, 1, 1 };
//int dy[] = { 0, 0, -1, 1, -1, 1, -1, 1 };

ld det(ld x1, ld y1, ld x2, ld y2, ld x3, ld y3)
{
	return x1 * y2 - x2 * y1 + x2 * y3 - x3 * y2 + x3 * y1 - x1 * y3;
}

const int MOD = 1000000007;

struct mod_int {
	int val;

	mod_int(long long v = 0) {
		if (v < 0)
			v = v % MOD + MOD;

		if (v >= MOD)
			v %= MOD;

		val = v;
	}

	static int mod_inv(int a, int m = MOD) {
		// https://en.wikipedia.org/wiki/Extended_Euclidean_algorithm#Example
		int g = m, r = a, x = 0, y = 1;

		while (r != 0) {
			int q = g / r;
			g %= r; swap(g, r);
			x -= q * y; swap(x, y);
		}

		return x < 0 ? x + m : x;
	}

	explicit operator int() const {
		return val;
	}

	mod_int& operator+=(const mod_int& other) {
		val += other.val;
		if (val >= MOD) val -= MOD;
		return *this;
	}

	mod_int& operator-=(const mod_int& other) {
		val -= other.val;
		if (val < 0) val += MOD;
		return *this;
	}

	static unsigned fast_mod(uint64_t x, unsigned m = MOD) {
		return x % m;
	}

	mod_int& operator*=(const mod_int& other) {
		val = fast_mod((uint64_t)val * other.val);
		return *this;
	}

	mod_int& operator/=(const mod_int& other) {
		return *this *= other.inv();
	}

	friend mod_int operator+(const mod_int& a, const mod_int& b) { return mod_int(a) += b; }
	friend mod_int operator-(const mod_int& a, const mod_int& b) { return mod_int(a) -= b; }
	friend mod_int operator*(const mod_int& a, const mod_int& b) { return mod_int(a) *= b; }
	friend mod_int operator/(const mod_int& a, const mod_int& b) { return mod_int(a) /= b; }

	mod_int& operator++() {
		val = val == MOD - 1 ? 0 : val + 1;
		return *this;
	}

	mod_int& operator--() {
		val = val == 0 ? MOD - 1 : val - 1;
		return *this;
	}

	mod_int operator++(int) { mod_int before = *this; ++* this; return before; }
	mod_int operator--(int) { mod_int before = *this; --* this; return before; }

	mod_int operator-() const {
		return val == 0 ? 0 : MOD - val;
	}

	bool operator==(const mod_int& other) const { return val == other.val; }
	bool operator!=(const mod_int& other) const { return val != other.val; }

	mod_int inv() const {
		return mod_inv(val);
	}

	mod_int pow(long long p) const {
		assert(p >= 0);
		mod_int a = *this, result = 1;

		while (p > 0) {
			if (p & 1)
				result *= a;

			a *= a;
			p >>= 1;
		}

		return result;
	}

	friend ostream& operator<<(ostream& stream, const mod_int& m) {
		return stream << m.val;
	}
};

vector<mod_int> inv, factorial, inv_factorial;

void prepare_factorials(int maximum) {
	inv.assign(maximum + 1, 1);

	// Make sure MOD is prime, which is necessary for the inverse algorithm below.
	for (int p = 2; p * p <= MOD; p++)
		assert(MOD % p != 0);

	for (int i = 2; i <= maximum; i++)
		inv[i] = inv[MOD % i] * (MOD - MOD / i);

	factorial.resize(maximum + 1);
	inv_factorial.resize(maximum + 1);
	factorial[0] = inv_factorial[0] = 1;

	for (int i = 1; i <= maximum; i++) {
		factorial[i] = i * factorial[i - 1];
		inv_factorial[i] = inv[i] * inv_factorial[i - 1];
	}
}


int n;
int a[MAXN];

int f[MAXN];
int father(int x)
{
	return f[x] == x ? x : f[x] = father(f[x]);
}
int sz[MAXN];

void solve()
{
	read(n);
	prepare_factorials(n);
	input(a + 1, a + n + 1);
	for (int i = 1; i <= n; ++i)
		f[i] = i;
	int loop = 0, k = 0;
	for (int i = 1; i <= n; ++i)
	{
		if (a[i] == -1)
		{
			k++;
			continue;
		}
		int x = father(i), y = father(a[i]);
		if (x == y) loop++;
		f[x] = y;
	}
	for (int i = 1; i <= n; ++i) sz[father(i)]++;
	mod_int res = loop * mod_int(n - 1).pow(k); // the number of loops already existing
	vector<mod_int> dp(n + 1);
	dp[0] = 1;
	for (int i = 1; i <= n; ++i)
	{
		if (a[i] != -1) continue;
		res += (sz[father(i)] - 1) * mod_int(n - 1).pow(k - 1); // the number of loops of length 1
		for (int j = n - 1; j >= 0; --j)
			dp[j + 1] += dp[j] * sz[father(i)];
	}
	for (int i = 2; i <= k; ++i)
	{
		res += mod_int(n - 1).pow(k - i) * dp[i] * factorial[i - 1];
	}
	res = n * mod_int(n - 1).pow(k) - res;
	writeln(int(res));
}

int main()
{

	int T = 1;
	//read(T);
	for (int test = 1; test <= T; ++test)
	{
		//write("Case #", test, ": ");
		solve();
	}

	return 0;
}

Measured memory_bytes: 3768