#include<bits/stdc++.h>
#define rep(i,a,b) for(int i=a;i<b;i++)
#define rrep(i,a,b) for(int i=a;i>=b;i--)
#define fore(i,a) for(auto &i:a)
#define all(x) (x).begin(),(x).end()
//#pragma GCC optimize ("-O3")
using namespace std; void _main(); int main() { cin.tie(0); ios::sync_with_stdio(false); _main(); }
typedef long long ll; const int inf = INT_MAX / 2; const ll infl = 1LL << 60;
template<class T>bool chmax(T& a, const T& b) { if (a < b) { a = b; return 1; } return 0; }
template<class T>bool chmin(T& a, const T& b) { if (b < a) { a = b; return 1; } return 0; }
//---------------------------------------------------------------------------------------------------
template<int MOD> struct ModInt {
    static const int Mod = MOD; unsigned x; ModInt() : x(0) { }
    ModInt(signed sig) { x = sig < 0 ? sig % MOD + MOD : sig % MOD; }
    ModInt(signed long long sig) { x = sig < 0 ? sig % MOD + MOD : sig % MOD; }
    int get() const { return (int)x; }
    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 = (unsigned long long)x * that.x % MOD; return *this; }
    ModInt &operator/=(ModInt that) { return *this *= that.inverse(); }
    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/(ModInt that) const { return ModInt(*this) /= that; }
    ModInt inverse() const { long long a = x, b = MOD, u = 1, v = 0;
        while (b) { long long t = a / b; a -= t * b; std::swap(a, b); u -= t * v; std::swap(u, v); }
        return ModInt(u); }
    bool operator==(ModInt that) const { return x == that.x; }
    bool operator!=(ModInt that) const { return x != that.x; }
    ModInt operator-() const { ModInt t; t.x = x == 0 ? 0 : Mod - x; return t; }
};
template<int MOD> ostream& operator<<(ostream& st, const ModInt<MOD> a) { st << a.get(); return st; };
template<int MOD> ModInt<MOD> operator^(ModInt<MOD> a, unsigned long long k) {
    ModInt<MOD> r = 1; while (k) { if (k & 1) r *= a; a *= a; k >>= 1; } return r; }
template<typename T, int FAC_MAX> struct Comb { vector<T> fac, ifac;
    Comb(){fac.resize(FAC_MAX,1);ifac.resize(FAC_MAX,1);rep(i,1,FAC_MAX)fac[i]=fac[i-1]*i;
        ifac[FAC_MAX-1]=T(1)/fac[FAC_MAX-1];rrep(i,FAC_MAX-2,1)ifac[i]=ifac[i+1]*T(i+1);}
    T aPb(int a, int b) { if (b < 0 || a < b) return T(0); return fac[a] * ifac[a - b]; }
    T aCb(int a, int b) { if (b < 0 || a < b) return T(0); return fac[a] * ifac[a - b] * ifac[b]; }
    T nHk(int n, int k) { if (n == 0 && k == 0) return T(1); if (n <= 0 || k < 0) return 0;
        return aCb(n + k - 1, k); } // nHk = (n+k-1)Ck : n is separator
    T pairCombination(int n) {if(n%2==1)return T(0);return fac[n]*ifac[n/2]/(T(2)^(n/2));}
    // combination of paris for n
}; 
typedef ModInt<1000000007> mint;
struct UnionFind {
	using T = int;
	const T def = 0;
	T f(T a, T b) { return a + b; }
	//==========================================
	vector<int> par;
	vector<T> value;
	UnionFind() {}
	UnionFind(int NV) { init(NV); }
	void init(int NV) { par.clear(); rep(i, 0, NV) par.push_back(i); value.resize(NV, 1); }
	void reset() { rep(i, 0, par.size()) par[i] = i; }
	int operator[](int x) {
		if (par[x] == x) return x;
		else {
			int res = operator[](par[x]);
			if (res != par[x]) {
				value[res] = f(value[res], value[par[x]]);
				value[par[x]] = def;
			}
			return par[x] = res;
		}
	}
	// uf(x,y)->y
	void operator()(int x, int y) {
		x = operator[](x); y = operator[](y);
		if (x != y) {
			value[y] += value[par[x]];
			value[par[x]] = def;
			par[x] = y;
		}
	}
	T getValues(int x) { return value[operator[](x)]; };
};
/*---------------------------------------------------------------------------------------------------
　　　　　　　　　　　 ∧＿∧
　　　　　 ∧＿∧ 　（´<_｀ ）　 Welcome to My Coding Space!
　　　　 （ ´_ゝ`）　/　 ⌒i     @hamayanhamayan0
　　　　／　　　＼　 　  |　|
　　　 /　　 /￣￣￣￣/　　|
　 ＿_(__ﾆつ/　    ＿/ .| .|＿＿＿＿
　 　　　＼/＿＿＿＿/　（u　⊃
---------------------------------------------------------------------------------------------------*/














int N, P[5010], K;
vector<int> gr;
//---------------------------------------------------------------------------------------------------
int cycle_num = 0;
bool pending[5010];
void unite() {
	UnionFind uf(N);
	int K = 0;
	rep(i, 0, N) {
		if (0 <= P[i]) {
			int x = i;
			int y = P[i];
			if (uf[x] != uf[y]) {
				uf(x, y);
			}
			else cycle_num++;
		}
		else K++;
	}

	rep(i, 0, N) if (P[i] < 0) pending[uf[i]] = true;
	rep(i, 0, N) if (i == uf[i] && pending[i]) gr.push_back(uf.getValues(i));
}
//---------------------------------------------------------------------------------------------------
Comb<mint, 10101> com;
mint dp[5010][5010];
bool vis[5010][5010];
mint calc() {
	dp[0][0] = 1;

	rep(i, 0, K) rep(len, 0, K) {
		dp[i + 1][len] += dp[i][len];
		dp[i + 1][len + 1] += dp[i][len] * gr[i];
	}

	mint res = 0;

	// cycle 1
	rep(i, 0, K) res += mint(gr[i] - 1) * (mint(N - 1) ^ (K - 1));

	// cycle greater than 1
	rep(len, 2, K + 1) res += dp[K][len] * com.fac[len - 1] * (mint(N - 1) ^ (K - len));

	return res;
}
//---------------------------------------------------------------------------------------------------
void _main() {
	cin >> N;
	rep(i, 0, N) {
		cin >> P[i], P[i]--;
		if (P[i] < 0) K++;
	}

	unite();

	mint ans = mint(N - cycle_num) * (mint(N - 1) ^ K);
	ans -= calc();

	cout << ans << endl;
}







Measured memory_bytes: 101912