#include <iostream>
#include <string>
#include <cmath>
#include<algorithm>
#include<stack>
#include<queue>
#include<map>
#include<set>
#include<iomanip>
#define _USE_MATH_DEFINES
#include <math.h>
#include <functional>
#include<complex>
using namespace std;

#define rep(i,x) for(ll i=0;i<x;i++)
#define repn(i,x) for(ll i=1;i<=x;i++)

typedef long long ll;
const ll INF = 1e17;
const ll MOD = 1000000007;
const ll MAX = 4000001;
const long double eps = 1E-14;

ll max(ll a, ll b) {
	if (a > b) { return a; }
	return b;
}

ll min(ll a, ll b) {
	if (a > b) { return b; }
	return a;
}

ll gcd(ll a, ll b) {
	if (b == 0) { return a; }
	if (a < b) { return gcd(b, a); }
	return gcd(b, a % b);
}

ll lcm(ll a, ll b) {
	return a / gcd(a, b) * b;
}

struct edge {
	ll ind;
	ll fr;
	ll to;
	ll d;
};

class mint {
	long long x;
public:
	mint(long long x = 0) : x((x% MOD + MOD) % MOD) {}
	mint operator-() const {
		return mint(-x);
	}
	mint& operator+=(const mint& a) {
		if ((x += a.x) >= MOD) x -= MOD;
		return *this;
	}
	mint& operator-=(const mint& a) {
		if ((x += MOD - a.x) >= MOD) x -= MOD;
		return *this;
	}
	mint& operator*=(const  mint& a) {
		(x *= a.x) %= MOD;
		return *this;
	}
	mint operator+(const mint& a) const {
		mint res(*this);
		return res += a;
	}
	mint operator-(const mint& a) const {
		mint res(*this);
		return res -= a;
	}
	mint operator*(const mint& a) const {
		mint res(*this);
		return res *= a;
	}

	mint pow(ll t) const {
		if (!t) return 1;
		mint a = pow(t >> 1);
		a *= a;
		if (t & 1) a *= *this;
		return a;
	}

	// for prime MOD
	mint inv() const {
		return pow(MOD - 2);
	}
	mint& operator/=(const mint& a) {
		return (*this) *= a.inv();
	}
	mint operator/(const mint& a) const {
		mint res(*this);
		return res /= a;
	}

	friend ostream& operator<<(ostream& os, const mint& m) {
		os << m.x;
		return os;
	}
};

mint pw(mint a, ll b) {
	if (b == 0) { return 1; }
	mint ret = pw(a, b >> 1);
	ret *= ret;
	if (b & 1) { ret *= a; }
	return ret;
}

typedef vector<ll> vll;
typedef vector<vector<ll>> vvll;
typedef vector<vector<vector<ll>>> vvvll;

typedef vector<mint> vmint;
typedef vector<vector<mint>> vvmint;
typedef vector<vector<vector<mint>>> vvvmint;

//////////////////////////////////////
struct unionfind {
	ll N;
	vector<ll> par;
	vector<ll> sz;//根の場合、その木のサイズ

	unionfind(ll N0) {
		N = N0;
		par.assign(N + 1, -1);
		sz.assign(N + 1, 1);

	}

	ll root(ll x) {
		if (par[x] == -1) { return x; }
		par[x] = root(par[x]);
		return par[x];
	}

	bool same(ll x, ll y) {
		return root(x) == root(y);
	}

	void unite(ll x, ll y) {
		ll a = root(x);
		ll b = root(y);
		if (a != b) {
			par[a] = b; sz[b] += sz[a];
		}

	}

	ll size(ll x) {
		ll r = root(x);
		return sz[r];
	}
};

int main() {
	ll N;
	cin >> N;
	vll P(N + 1);
	repn(i, N) cin >> P[i];

	unionfind uf(N);
	repn(i, N) {
		if (P[i] != -1) { uf.unite(i, P[i]); }
	}

	vll sz(N + 1, 0);
	vll c(N + 1, 0);
	ll K = 0;
	ll M = 0;
	repn(i, N) {
		if (uf.root(i) == i) { sz[i] = uf.size(i); M++; }
		if (P[i] == -1) { c[uf.root(i)] = 1; K++; }
	}
	
	vll con;
	repn(i, N) { if (c[i] == 1) { con.push_back(sz[i]);} }

	vvmint dp(K + 1, vmint(K + 1, 0));

	//rep(i, K) cout << con[i] << endl;

	dp[0][0] = 1;
	repn(i, K)rep(j, K+1) {
		dp[i][j] = dp[i - 1][j];
		if (j > 0)dp[i][j] += dp[i - 1][j - 1] * con[i - 1];
		//cout << i <<" "<< j <<" "<< dp[i][j] << endl;
	}

	vmint fr(K + 1, 1);
	repn(i, K) fr[i] = fr[i - 1] * i;

	mint cycle = 0;
	repn(i, K) {
		if (i == 1) { dp[K][i]=dp[K][i]-K; }
		//cout << dp[K][i] << endl;
		cycle += dp[K][i] * fr[i-1] * pw(N-1,K-i);
	}

	//cout << M << " " << K << " " << cycle << endl;

	mint consum = pw(N - 1, K) * (M - K) + cycle;

	//cout <<pow(N-1,K)*(N-1)<<endl;

	mint ans = pw(N - 1, K) * (N - (M-K)) - cycle;

	cout << ans << endl;

}

Measured memory_bytes: 199060