#include <bits/stdc++.h>
#include <sys/types.h>
#include <unistd.h>

#define _overload(_1,_2,_3,name,...) name
#define _rep(i,n) _range(i,0,n)
#define _range(i,a,b) for(int i=int(a);i<int(b);++i)
#define rep(...) _overload(__VA_ARGS__,_range,_rep,)(__VA_ARGS__)

#define _rrep(i,n) _rrange(i,n,0)
#define _rrange(i,a,b) for(int i=int(a)-1;i>=int(b);--i)
#define rrep(...) _overload(__VA_ARGS__,_rrange,_rrep,)(__VA_ARGS__)

#define _all(arg) begin(arg),end(arg)
#define uniq(arg) sort(_all(arg)),(arg).erase(unique(_all(arg)),end(arg))
#define getidx(ary,key) lower_bound(_all(ary),key)-begin(ary)
#define clr(a,b) memset((a),(b),sizeof(a))
#define bit(n) (1LL<<(n))
#define popcount(n) (__builtin_popcountll(n))

using namespace std;

template<class T>bool chmax(T &a, const T &b) { return (a<b)?(a=b,1):0;}
template<class T>bool chmin(T &a, const T &b) { return (b<a)?(a=b,1):0;}

using ll=long long;

class ModInt {
public:
	static unsigned MOD;
	ModInt(): x(0) {}
	ModInt(signed y) : x(y >= 0 ? y % MOD : MOD - (-y) % MOD) {}
	ModInt(signed long long y) : x(y >= 0 ? y % MOD : MOD - (-y) % MOD) {}


	// Arithmetic Oprators
	ModInt &operator+=(ModInt that) {
		if ((x += that.x) >= MOD) x -= MOD; 
		return *this;
	}
	ModInt &operator-=(ModInt that) {
		if ((x += MOD - that.x) >= MOD) x -= MOD; 
		return *this;
	}
	ModInt &operator*=(ModInt that) {
		x = 1LL * x * that.x % MOD;
		return *this;
	}

	ModInt &operator+=(const int that) { return *this += ModInt(that);}
	ModInt &operator-=(const int that) { return *this -= ModInt(that);}
	ModInt &operator*=(const int that) { return *this *= ModInt(that);}

	// Comparators
	bool operator <(ModInt that) { return x < that.x; }
	bool operator >(ModInt that) { return x > that.x; }
	bool operator<=(ModInt that) { return x <= that.x; }
	bool operator>=(ModInt that) { return x >= that.x; }
	bool operator!=(ModInt that) { return x != that.x; }
	bool operator==(ModInt that) { return x == that.x; }

	// Utilities
	unsigned getval() const { return x;}
	ModInt operator+(ModInt that) const { return ModInt(*this) += that;}
	ModInt operator-(ModInt that) const { return ModInt(*this) -= that;}
	ModInt operator*(ModInt that) const { return ModInt(*this) *= that;}
	ModInt operator+(const int that) const { return ModInt(*this) += that;}
	ModInt operator-(const int that) const { return ModInt(*this) -= that;}
	ModInt operator*(const int that) const { return ModInt(*this) *= that;}
	ModInt operator=(const int that) { return *this = ModInt(that);}
	friend istream &operator>>(istream& is, ModInt &that) {ll tmp; is >> tmp; that = ModInt(tmp);return is;}
	friend ostream &operator<<(ostream& os, const ModInt &that) { return os << that.x; }
	
	ModInt power(ll n) const {
		ll b = 1LL, a = x;
		while(n){
			if(n & 1) b = b * a % MOD;
			a = a * a % MOD;
			n >>= 1;
		}
		return ModInt(b);
	}
private:
	unsigned x;
};

unsigned ModInt::MOD = 1000000007;
using mint = ModInt;
const mint ZERO = mint(0);
const mint ONE = mint(1);
const mint TWO = mint(2);

// Description: 素集合を管理するデータ構造
// TimeComplexity: 初期化$\mathcal{O}(n)$ 更新$\mathcal{O}(\log n)$
// Verifyed: AOJ DSL_1_A

struct Union_find {
	Union_find(int n) {data.assign(n, -1);}
	int find(int x) {return (data[x] < 0) ? x : data[x] = find(data[x]);}
	void unite(int a, int b) {
		a = find(a), b = find(b);
		if (a != b) {
			if (data[a] < data[b]) swap(a, b);
			data[a] += data[b], data[b] = a;
		}
	}
	bool same(int a, int b) {return find(a) == find(b);}
	int size(int x) {return -data[find(x)];}
	vector<int> data;
};

map<int, int> memo;
vector<int> sz;
mint dp[5010][5010];
mint num[5010];
mint fact[5010];

int main(void){
	int n;
	cin >> n;
	vector<int> p(n);
	rep(i, n) cin >> p[i];

	fact[0] = ONE;
	rep(i, 1, n + 1) fact[i] = mint(i) * fact[i - 1];

	Union_find uf(n);

	int loop = 0;

	rep(i, n) {
		if(p[i] != -1) {
			if (uf.same(i, p[i] - 1)) {
				loop++;
			}
			uf.unite(i, p[i] - 1);
		}
	}

	rep(i, n) {
		if(p[i] == -1) {
			memo[uf.find(i)]++;
		}
	}

	for(auto &elem: memo) {
		sz.push_back(uf.size(elem.first));
	}

	const int k = sz.size();
	const mint all = mint(n - 1).power(k);
	
	mint ans = all * mint(n - loop);

	dp[0][0] = ONE;
	rep(i, k) rep(j, i + 1) {
		if(dp[i][j] == ZERO) continue;
		dp[i + 1][j] += dp[i][j];
		dp[i + 1][j + 1] += dp[i][j] * mint(sz[i]);
	}

	rep(i, k) {
		mint cur = mint(sz[i] - 1) * mint(n - 1).power(k - 1);
		ans -= cur;
	}

	rep(j, 2, k + 1) {
		mint cur = dp[k][j] * fact[j - 1] * mint(n - 1).power(k - j);
		ans -= cur;
	}

	cout << ans << endl;
	return 0;
}

Measured memory_bytes: 102016