#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for(int i = 0; i < n; i++)
#define rep2(i, x, n) for(int i = x; i <= n; i++)
#define rep3(i, x, n) for(int i = x; i >= n; i--)
#define elif else if
#define sp(x) fixed << setprecision(x)
#define pb push_back
#define eb emplace_back
#define all(x) x.begin(), x.end()
#define sz(x) (int)x.size()
using ll = long long;
using ld = long double;
using pii = pair<int, int>;
using pil = pair<int, ll>;
using pli = pair<ll, int>;
using pll = pair<ll, ll>;
const ll MOD = 1e9+7;
//const ll MOD = 998244353;
const int inf = (1<<30)-1;
const ll INF = (1LL<<60)-1;
const ld EPS = 1e-10;
template<typename T> bool chmax(T &x, const T &y) {return (x < y)? (x = y, true) : false;};
template<typename T> bool chmin(T &x, const T &y) {return (x > y)? (x = y, true) : false;};

template<ll mod> struct Mod_Int{
    ll x;
    Mod_Int() {}
    Mod_Int(ll y) : x (y >= 0 ? y % mod : (mod - (-y) % mod) % mod) {}

    Mod_Int &operator += (const Mod_Int &p) {
        x = (x + p.x) % mod;
        return *this;
    }

    Mod_Int &operator -= (const Mod_Int &p) {
        x = (x + mod - p.x) % mod;
        return *this;
    }

    Mod_Int &operator *= (const Mod_Int &p) {
        x = (x * p.x) % mod;
        return *this;
    }

    Mod_Int &operator /= (const Mod_Int &p) {
        *this *= p.inverse();
        return *this;
    }

    Mod_Int &operator ++ () {return *this += (Mod_Int)1;}

    Mod_Int operator ++ (int) {
        Mod_Int tmp = *this;
        ++*this;
        return tmp;
    }

    Mod_Int &operator -- () {return *this -= (Mod_Int)1;}

    Mod_Int operator -- (int) {
        Mod_Int tmp = *this;
        --*this;
        return tmp;
    }

    Mod_Int operator - () const {return Mod_Int(-x);}

    Mod_Int operator + (const Mod_Int &p) const {return Mod_Int(*this) += p;}

    Mod_Int operator - (const Mod_Int &p) const {return Mod_Int(*this) -= p;}

    Mod_Int operator * (const Mod_Int &p) const {return Mod_Int(*this) *= p;}

    Mod_Int operator / (const Mod_Int &p) const {return Mod_Int(*this) /= p;}

    bool operator == (const Mod_Int &p) const {return x == p.x;}

    bool operator != (const Mod_Int &p) const {return x != p.x;}

    Mod_Int pow (ll n) const {
        Mod_Int now = *this, ret = 1;
        while(n > 0){
            if(n & 1) ret *= now;
            now *= now, n >>= 1;
        }
        return ret;
    }

    Mod_Int inverse () const {
        return pow(mod-2);
    }
};

using mint = Mod_Int<MOD>;
const int MAX_N = 1e6;
mint fac[MAX_N+1], ifac[MAX_N+1];

void init(){
    fac[0] = 1;
    rep2(i, 1, MAX_N){
        fac[i] = fac[i-1]*i;
    }
    ifac[MAX_N] = fac[MAX_N].inverse();
    rep3(i, MAX_N, 1){
        ifac[i-1] = ifac[i]*i;
    }
}

mint comb(int n, int k){
    return fac[n]*ifac[n-k]*ifac[k];
}

mint perm(int n, int k){
    return fac[n]*ifac[n-k];
}

struct Union_Find_Tree{
    vector<int> par, rank, num;
    //要素数N(0〜N-1)の初期化されたUnion-Find木を構築
    Union_Find_Tree(int N){
        par.assign(N, -1);
        rank.assign(N, 0);
        num.assign(N, 1);
    }
    //ノードxの根を取得
    int root(int x){
        if(par[x] < 0 || par[x] == x) return x;
        return par[x] = root(par[x]);
    }
    //xとyの属する集合を併合
    void unite(int x, int y){
        x = root(x), y = root(y);
        if(x == y) return;
        elif(rank[x] < rank[y]) par[x] = y, num[y] += num[x];
        else{
            par[y] = x, num[x] += num[y];
            if(rank[x] == rank[y]) rank[x]++;
        }
    }
    //xの属する集合の要素数を取得
    int size(int x){
        return num[root(x)];
    }
    //xとyが同じ集合に属するか判定
    bool same(int x, int y){
        return root(x) == root(y);
    }
    //初期化する
    void clear(){
        fill(par.begin(), par.end(), -1);
        fill(rank.begin(), rank.end(), 0);
        fill(num.begin(), num.end(), 1);
    }
};

int main(){
    int N;
    cin >> N;
    int P[N];
    Union_Find_Tree uft(N);
    int cnt = 0;
    rep(i, N){
        cin >> P[i];
        if(P[i] != -1){
            P[i]--;
            if(uft.same(i, P[i])) cnt++;
            else uft.unite(i, P[i]);
        }
    }
    init();
    mint pw[N+1];
    pw[0] = 1;
    rep(i, N) pw[i+1] = pw[i]*(N-1);
    vector<mint> sz;
    rep(i, N){
        if(P[i] == -1) sz.pb(uft.size(i));
    }
    int n = sz(sz);
    mint dp[n+1][n+1];
    fill(dp[0], dp[n+1], 0);
    dp[0][0] = 1;
    rep(i, n){
        rep(j, n){
            dp[i+1][j] += dp[i][j];
            dp[i+1][j+1] += sz[i]*dp[i][j];
        }
    }
    mint ans = (mint)(N-cnt)*pw[n];
    rep2(i, 1, n){
        ans -= dp[n][i]*fac[i-1]*pw[n-i];
    }
    ans += (mint)n*pw[n-1];
    cout << ans.x << endl;
}

Measured memory_bytes: 214824