#include <bits/stdc++.h>
#define ADD(a, b) a = (a + ll(b)) % mod
#define MUL(a, b) a = (a * ll(b)) % mod
#define MAX(a, b) a = max(a, b)
#define MIN(a, b) a = min(a, b)
#define rep(i, a, b) for(int i = int(a); i < int(b); i++)
#define rer(i, a, b) for(int i = int(a) - 1; i >= int(b); i--)
#define all(a) (a).begin(), (a).end()
#define sz(v) (int)(v).size()
#define pb push_back
#define sec second
#define fst first
#define debug(fmt, ...) Debug(__LINE__, ":", fmt, ##__VA_ARGS__)
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> pi;
typedef pair<ll, ll> pl;
typedef pair<int, pi> ppi;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<vl> mat;
typedef complex<double> comp;
void Debug() {cerr << '\n'; }
template<class FIRST, class... REST>void Debug(FIRST arg, REST... rest){
	cerr<<arg<<" ";Debug(rest...);}
template<class T>ostream& operator<<(ostream& out,const vector<T>& v) {
	out<<"[";if(!v.empty()){rep(i,0,sz(v)-1)out<<v[i]<<", ";out<<v.back();}out<<"]";return out;}
template<class S, class T>ostream& operator<<(ostream& out,const pair<S, T>& v){
	out<<"("<<v.first<<", "<<v.second<<")";return out;}
const int MAX_N = 500010;
const int MAX_V = 100010;
const double eps = 1e-6;
const ll mod = 1000000007;
const int inf = (1 << 30) - 1;
const ll linf = 1LL << 60;
const double PI = 3.14159265358979323846;
mt19937 rng; //use it by rng() % mod, shuffle(all(vec), rng)
///////////////////////////////////////////////////////////////////////////////////////////////////


int N;
ll dp[5010][5010];
ll powN[5010];
vi G[MAX_N];
int A[MAX_N];
bool used[MAX_N];
vi D; int L; //renketsu, left

pi loop(int v) {
	used[v] = true;
	int res = 1;
	int found = (A[v] == -1);
	rep(i, 0, sz(G[v])) {
		int n = G[v][i];
		if(!used[n]) {
			pi p = loop(n);
			res += p.fst;
			found |= p.sec;
		}
	}
	return pi(res, found);
}

void solve() {
	cin >> N;
	int nos = 0;
	powN[0] = 1;
	rep(i, 0, N + 1) {
		powN[i + 1] = powN[i] * (N - 1) % mod;
	}
	rep(i, 0, N) {
		cin >> A[i]; A[i] = max(A[i] - 1, -1);
		if(A[i] != -1) {
			G[i].pb(A[i]);
			G[A[i]].pb(i);
		}
		else nos++;
	}
	L = 0;
	ll ans = 0;
	rep(i, 0, N) {
		if(!used[i]) {
			pi p = loop(i);
			if(!p.sec) L += p.fst;
			else D.pb(p.fst);
			ADD(ans, powN[nos] * (p.fst - 1) % mod);
		}
	}
	// debug(ans, D);
	dp[0][0] = 1;
	rep(i, 0, nos) {
		rep(j, 0, i + 1) {
			if(dp[i][j] == 0) continue;
			ADD(dp[i + 1][j], dp[i][j]);
			ADD(dp[i + 1][j + 1], dp[i][j] * D[i] % mod * (j + 1) % mod);
		}
		ll cycle = 0;
		rep(j, 0, i + 1) {
			ADD(cycle, dp[i][j] % mod * D[i] % mod * powN[i - j] % mod);
			// debug(i, j, cycle);
		}
		ADD(ans, (mod + powN[i + 1] - cycle + powN[i]) * powN[nos - i - 1] % mod);
	}
	cout << ans << "\n";
}

uint32_t rd() {
	uint32_t res;
#ifdef __MINGW32__
	asm volatile("rdrand %0" :"=a"(res) ::"cc");
#else
	res = std::random_device()();
#endif
	return res;
}

int main() {
#ifndef LOCAL
	ios::sync_with_stdio(false);
    cin.tie(0);
#endif
    cout << fixed;
	cout.precision(20);
    cerr << fixed;
	cerr.precision(6);
	rng.seed(rd());
#ifdef LOCAL
	//freopen("in.txt", "wt", stdout); //for tester
	if(!freopen("in.txt", "rt", stdin)) return 1;
#endif	
	solve();
    cerr << "Time: " << 1.0 * clock() / CLOCKS_PER_SEC << " s.\n";
	return 0;
}



Measured memory_bytes: 132232