//#define _DEBUG
#include "bits/stdc++.h"
#define CHOOSE(a) CHOOSE2 a
#define CHOOSE2(a0,a1,a2,a3,a4,x,...) x
#define debug_1(x1) cout<<#x1<<": "<<x1<<endl
#define debug_2(x1,x2) cout<<#x1<<": "<<x1<<", "#x2<<": "<<x2<<endl
#define debug_3(x1,x2,x3) cout<<#x1<<": "<<x1<<", "#x2<<": "<<x2<<", "#x3<<": "<<x3<<endl
#define debug_4(x1,x2,x3,x4) cout<<#x1<<": "<<x1<<", "#x2<<": "<<x2<<", "#x3<<": "<<x3<<", "#x4<<": "<<x4<<endl
#define debug_5(x1,x2,x3,x4,x5) cout<<#x1<<": "<<x1<<", "#x2<<": "<<x2<<", "#x3<<": "<<x3<<", "#x4<<": "<<x4<<", "#x5<<": "<<x5<<endl
#ifdef _DEBUG
#define debug(...) CHOOSE((__VA_ARGS__,debug_5,debug_4,debug_3,debug_2,debug_1,~))(__VA_ARGS__)
#else
#define debug(...)
#endif
#define rep(index,num) for(int index=0;index<(int)num;index++)
#define rep1(index,num) for(int index=1;index<=(int)num;index++)
#define brep(index,num) for(int index=(int)num-1;index>=0;index--)
#define brep1(index,num) for(int index=(int)num;index>0;index--)
#define scan(argument) cin>>argument
#define prin(argument) cout<<argument<<endl
#define kaigyo cout<<endl
#define eps 1e-7
#define mp(a1,a2) make_pair(a1,a2)
#define ALL(a) (a).begin(),(a).end()
#define rALL(a) (a).rbegin(),(a).rend()
typedef long long ll;
typedef long double ld;
using namespace std;
typedef pair<ll,ll> pll;
typedef pair<int,int> pint;
typedef vector<int> vint;
typedef vector<ll> vll;
typedef vector<pint> vpint;
typedef vector<pll> vpll;
ll INFl=(ll)1e+18+1;
int INF=1e+9+1;

template<unsigned int mod_>
struct ModInt{
	using uint = unsigned int;
	using ll = long long;
	using ull = unsigned long long;

	constexpr static uint mod = mod_;

	uint v;
	ModInt():v(0){}
	ModInt(ll _v):v(normS(_v%mod+mod)){}
	explicit operator bool() const {return v!=0;}
	static uint normS(const uint &x){return (x<mod)?x:x-mod;}		// [0 , 2*mod-1] -> [0 , mod-1]
	static ModInt make(const uint &x){ModInt m; m.v=x; return m;}
	ModInt operator+(const ModInt& b) const { return make(normS(v+b.v));}
	ModInt operator-(const ModInt& b) const { return make(normS(v+mod-b.v));}
	ModInt operator-() const { return make(normS(mod-v)); }
	ModInt operator*(const ModInt& b) const { return make((ull)v*b.v%mod);}
	ModInt operator/(const ModInt& b) const { return *this*b.inv();}
	ModInt& operator+=(const ModInt& b){ return *this=*this+b;}
	ModInt& operator-=(const ModInt& b){ return *this=*this-b;}
	ModInt& operator*=(const ModInt& b){ return *this=*this*b;}
	ModInt& operator/=(const ModInt& b){ return *this=*this/b;}
	ModInt& operator++(int){ return *this=*this+1;}
	ModInt& operator--(int){ return *this=*this-1;}
	ll extgcd(ll a,ll b,ll &x,ll &y) const{
		ll p[]={a,1,0},q[]={b,0,1};
		while(*q){
			ll t=*p/ *q;
			rep(i,3) swap(p[i]-=t*q[i],q[i]);
		}
		if(p[0]<0) rep(i,3) p[i]=-p[i];
		x=p[1],y=p[2];
		return p[0];
	}
	ModInt inv() const {
		ll x,y;
		extgcd(v,mod,x,y);
		return make(normS(x+mod));
	}
	ModInt pow(ll p) const {
		if(p<0) return inv().pow(-p);
		ModInt a = 1;
		ModInt x = *this;
		while(p){
			if(p&1) a *= x;
			x *= x;
			p >>= 1;
		}
		return a;
	}
	bool operator==(const ModInt& b) const { return v==b.v;}
	bool operator!=(const ModInt& b) const { return v!=b.v;}
	friend istream& operator>>(istream &o,ModInt& x){
		ll tmp;
		o>>tmp;
		x=ModInt(tmp);
		return o;
	}
	friend ostream& operator<<(ostream &o,const ModInt& x){ return o<<x.v;}
};
using mint = ModInt<1000000007>;
struct UnionFind{
  vector<int> par;
  vector<int> sizes;
  vector<int> minus;
  UnionFind(int n) : par(n), sizes(n, 1), minus(n,0) {
    rep(i,n) par[i] = i;
  }
  int root(int x) {
    if (x == par[x]) return x;
    return par[x] = root(par[x]);
  }
  void unite(int x, int y) {
    x = root(x);
    y = root(y);
    if (x == y) return;
    if (sizes[x] < sizes[y]) swap(x, y);
    par[y] = x;
    sizes[x] += sizes[y];
	minus[x]|=minus[y];
  }
  bool same(int x, int y) {
    return root(x) == root(y);
  }
  int size(int x) {
    return sizes[root(x)];
  }
  bool ifminus(int x){
	  return minus[root(x)];
  }
  void tominus(int x){
	  minus[root(x)]=1;
  }
};
const int MAX = 200000;
mint fac[MAX], finv[MAX], inv[MAX];
void COMinit(){
    fac[0] = fac[1] = 1;
    finv[0] = finv[1] = 1;
    inv[1] = 1;
    for(int i = 2; i < MAX; i++){
        fac[i] = fac[i-1] * i;
        inv[i] = (mint)1/i;
        finv[i] = finv[i-1] * inv[i];
    }
}
int main(){
	COMinit();
	int N,P[5002];
	int K=0;
	scan(N);
	UnionFind uf(N);
	vint S;//-1を含む連結成分の要素数
	int notminus=0;
	rep(i,N){
		scan(P[i]);
		P[i]--;
		if(P[i]>=0) uf.unite(i,P[i]);
		else{
			K++;
			uf.tominus(i);
		}
	}
	rep(i,N){
		if(i==uf.root(i)){
			if(uf.ifminus(i)){
				S.push_back(uf.size(i));
			}
			else notminus++;
		}
	}
	rep(i,S.size()) debug(S[i]);
	mint Nm1th[5005];
	Nm1th[0]=1;
	rep(i,N) Nm1th[i+1]=Nm1th[i]*(N-1);
	mint ans=(mint)(N-notminus)*Nm1th[K];
	debug(K,notminus,ans);
	int Snum=S.size();
	mint dp[Snum+1][Snum+1]={};
	dp[0][0]=1;
	rep(i,Snum){
		rep(j,i+1){
			dp[i+1][j]+=dp[i][j];
			dp[i+1][j+1]+=dp[i][j]*S[i];
		}
	}
	rep(i,Snum+1){
		rep(j,Snum+1){
			debug(i,j,dp[i][j]);
		}
	}
	rep(i,Snum) ans-=(mint)(S[i]-1)*Nm1th[Snum-1];
	for(int k=2;k<=Snum;k++){
		ans-=dp[Snum][k]*fac[k-1]*Nm1th[Snum-k];
	}
	prin(ans);
	return 0;
}


Measured memory_bytes: 103628