#include <bits/stdc++.h>
#include <random>

using namespace std;

typedef unsigned long long ull;
typedef long long ll;
typedef double ld;
//#define ll __int128
//#define int ll
//#define int ll
//#define char ll
#define double ld
typedef vector<char> vc;
typedef vector<vc> vvc;
typedef vector<vvc> vvvc;
typedef pair<int, int> pii;
typedef pair<pii, pii> piii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;
typedef vector<pii> vpi;
typedef vector<vi> vvi;
typedef vector<vvi> vvvi;
typedef vector<short> vs;
typedef vector<vs> vvs;
typedef vector<vvs> vvvs;
typedef vector<ll> vl;
typedef vector<vl> vvl;
typedef vector<vvl> vvvl;
typedef vector<ld> vld;
typedef vector<vld> vvld;
typedef vector<vvld> vvvld;
typedef vector<string> vst;
typedef vector<vst> vvst;
typedef pair<ld, ld> pld;

#define inmin(a, b) a = min(a, (b))
#define inmax(a, b) a = max(a, (b))
#define ALL(a) a.begin(),a.end()
#define RALL(a) a.rbegin(),a.rend()
#define sqr(x) ((x) * (x))
#define fori(i, n) for(int i = 0; i < int(n); ++i)
#define SZ(a) ((int)((a).size()))
#define triple(T) tuple<T, T, T>
#define quad(T) tuple<T, T, T, T>
#define watch(x) cerr << (#x) << " = " << (x) << endl;

#ifdef MAX_HOME
#define cerr cout
#else
#define cerr if (false) cerr
#endif

const double PI = 2 * acos(0.0);
#define rand shittttty_shit
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
mt19937_64 rng_64(chrono::steady_clock::now().time_since_epoch().count());

const string DIGITS = "0123456789";
const string ALPH = "abcdefghijklmnopqrstuvwxyz";


template<class T0, class T1>
inline ostream &operator<<(ostream &out, pair<T0, T1> &a) {
    return out << "{" << a.first << ", " << a.second << "}";
}

template<class T0, class T1>
inline istream &operator>>(istream &in, pair<T0, T1> &a) {
    return in >> a.first >> a.second;
}

template<class T0, class T1, class T2>
inline ostream &operator<<(ostream &out, tuple<T0, T1, T2> &a) {
    return out << "{" << get<0>(a) << ", " << get<1>(a) << ", " << get<2>(a) << "}";
}

template<class T0, class T1, class T2, class T3>
inline ostream &operator<<(ostream &out, tuple<T0, T1, T2, T3> &a) {
    return out << "{" << get<0>(a) << ", " << get<1>(a) << ", " << get<2>(a) << ", " << get<3>(a) << "}";
}

template<class T>
inline ostream &operator<<(ostream &out, vector<T> &a) {
    out << "[";
    fori (i, a.size())out << a[i] << vector<string>{", ", "]  "}[i + 1 == a.size()];
    return out;
}


void smain();


signed main() {
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
#ifdef MAX_HOME
    freopen("input.txt", "r", stdin);
    clock_t start = clock();
#endif
    cout << setprecision(12) << fixed;
    smain();
#ifdef MAX_HOME
    cout << "\n\nTOTAL EXECUTION TIME: " << float(clock() - start) / CLOCKS_PER_SEC << endl;
#endif
    return 0;
}

const int M = 1e9 + 7;

int pw(int a, int n = M - 2) {
    int ret = 1;
    while (n) {
        if (n & 1)
            ret = (ll) ret * a % M;
        a = (ll) a * a % M;
        n >>= 1;
    }
    return ret;
}

const int N = 5050;

int dp[N][N];


vi g[N];
int p[N];
bool used[N];

bool have;
int siz = 0;

void dfs(int v) {
    used[v] = true;
    siz++;
    if (p[v] == -1)
        have = true;
    for (int to : g[v]) {
        if (used[to])
            continue;
        dfs(to);
    }
}

void insum(int & a, int b) {
    a = (a + b) % M;
}

void insub(int & a, int b) {
    a = (a - b + M) % M;
}

int fact[N];

void smain() {

    fact[0] = 1;
    for (int i = 1; i < N; ++i) {
        fact[i] = (ll) fact[i - 1] * i % M;
    }
    int n;
    cin >> n;
    for (int i = 1; i <= n; ++i) {
        cin >> p[i];
        if (p[i] != -1) {
            g[p[i]].push_back(i);
            g[i].push_back(p[i]);
        }
    }

    vi obj;

    int add = 0;

    int haves = 0;

    for (int i = 1; i <= n; ++i) {
        if (used[i])
            continue;
        have = false;
        siz = 0;
        dfs(i);
        add += siz - 1;
        if (have) {
            obj.push_back(siz);
            haves++;
        }
    }

    add += haves;
//    watch(add);
    int ans = (ll) pw(n - 1, haves) * add % M;

    dp[0][0] = 1;

    for (int i = 0; i < SZ(obj); ++i) {
        for (int cnt = 0; cnt < N; ++cnt) {
            insum(dp[i + 1][cnt], dp[i][cnt]);
            insum(dp[i + 1][cnt + 1], (ll) dp[i][cnt] * obj[i] % M);
        }
    }

    for (int cnt = 2; cnt <= haves; ++cnt) {
        int ways = (ll) dp[SZ(obj)][cnt] * fact[cnt - 1] % M * pw(n - 1, haves - cnt) % M;
        insub(ans, ways);
    }

    for (auto s : obj) {
        insub(ans, (ll) (s - 1) * pw(n - 1, haves - 1) % M);
    }

    cout << ans;
}

Measured memory_bytes: 102416