#include <algorithm>
#include <array>
#include <cassert>
#include <iostream>
#include <vector>
using namespace std;

template<typename T> ostream& operator<<(ostream &os, const vector<T> &v) { os << '{'; string sep; for (const auto &x : v) os << sep << x, sep = ", "; return os << '}'; }
template<typename T, size_t size> ostream& operator<<(ostream &os, const array<T, size> &arr) { os << '{'; string sep; for (const auto &x : arr) os << sep << x, sep = ", "; return os << '}'; }
template<typename A, typename B> ostream& operator<<(ostream &os, const pair<A, B> &p) { return os << '(' << p.first << ", " << p.second << ')'; }

void dbg_out() { cerr << endl; }
template<typename Head, typename... Tail> void dbg_out(Head H, Tail... T) { cerr << ' ' << H; dbg_out(T...); }

#ifdef NEAL_DEBUG
#define dbg(...) cerr << "(" << #__VA_ARGS__ << "):", dbg_out(__VA_ARGS__)
#else
#define dbg(...)
#endif

template<const int &MOD>
struct _m_int {
    int val;

    _m_int(int64_t v = 0) {
        if (v < 0) v = v % MOD + MOD;
        if (v >= MOD) v %= MOD;
        val = int(v);
    }

    static int inv_mod(int a, int m = MOD) {
        // https://en.wikipedia.org/wiki/Extended_Euclidean_algorithm#Example
        int g = m, r = a, x = 0, y = 1;

        while (r != 0) {
            int q = g / r;
            g %= r; swap(g, r);
            x -= q * y; swap(x, y);
        }

        return x < 0 ? x + m : x;
    }

    explicit operator int() const { return val; }
    explicit operator int64_t() const { return val; }

    _m_int& operator+=(const _m_int &other) {
        val -= MOD - other.val;
        if (val < 0) val += MOD;
        return *this;
    }

    _m_int& operator-=(const _m_int &other) {
        val -= other.val;
        if (val < 0) val += MOD;
        return *this;
    }

    static unsigned fast_mod(uint64_t x, unsigned m = MOD) {
#if !defined(_WIN32) || defined(_WIN64)
        return unsigned(x % m);
#endif
        // Optimized mod for Codeforces 32-bit machines.
        // x must be less than 2^32 * m for this to work, so that x / m fits in a 32-bit unsigned int.
        unsigned x_high = unsigned(x >> 32), x_low = unsigned(x);
        unsigned quot, rem;
        asm("divl %4\n"
            : "=a" (quot), "=d" (rem)
            : "d" (x_high), "a" (x_low), "r" (m));
        return rem;
    }

    _m_int& operator*=(const _m_int &other) {
        val = fast_mod(uint64_t(val) * other.val);
        return *this;
    }

    _m_int& operator/=(const _m_int &other) {
        return *this *= other.inv();
    }

    friend _m_int operator+(const _m_int &a, const _m_int &b) { return _m_int(a) += b; }
    friend _m_int operator-(const _m_int &a, const _m_int &b) { return _m_int(a) -= b; }
    friend _m_int operator*(const _m_int &a, const _m_int &b) { return _m_int(a) *= b; }
    friend _m_int operator/(const _m_int &a, const _m_int &b) { return _m_int(a) /= b; }

    _m_int& operator++() {
        val = val == MOD - 1 ? 0 : val + 1;
        return *this;
    }

    _m_int& operator--() {
        val = val == 0 ? MOD - 1 : val - 1;
        return *this;
    }

    _m_int operator++(int) { _m_int before = *this; ++*this; return before; }
    _m_int operator--(int) { _m_int before = *this; --*this; return before; }

    _m_int operator-() const {
        return val == 0 ? 0 : MOD - val;
    }

    bool operator==(const _m_int &other) const { return val == other.val; }
    bool operator!=(const _m_int &other) const { return val != other.val; }

    _m_int inv() const {
        return inv_mod(val);
    }

    _m_int pow(int64_t p) const {
        if (p < 0)
            return inv().pow(-p);

        _m_int a = *this, result = 1;

        while (p > 0) {
            if (p & 1)
                result *= a;

            p >>= 1;

            if (p > 0)
                a *= a;
        }

        return result;
    }

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

extern const int MOD = 1e9 + 7;
using mod_int = _m_int<MOD>;

struct union_find {
    // When data[x] < 0, x is a root and -data[x] is its tree size. When data[x] >= 0, data[x] is x's parent.
    vector<int> data;
    int components = 0;

    union_find(int n = -1) {
        if (n >= 0)
            init(n);
    }

    void init(int n) {
        data.assign(n + 1, -1);
        components = n;
    }

    int find(int x) {
        return data[x] < 0 ? x : data[x] = find(data[x]);
    }

    int get_size(int x) {
        return -data[find(x)];
    }

    bool unite(int x, int y) {
        x = find(x);
        y = find(y);

        if (x == y)
            return false;

        if (-data[x] < -data[y])
            swap(x, y);

        data[x] += data[y];
        data[y] = x;
        components--;
        return true;
    }
};


int main() {
    ios::sync_with_stdio(false);
#ifndef NEAL_DEBUG
    cin.tie(nullptr);
#endif

    int N;
    cin >> N;
    vector<int> P(N);
    union_find UF(N);

    for (int &p : P) {
        cin >> p;

        if (p >= 0)
            p--;
    }

    for (int i = 0; i < N; i++)
        if (P[i] >= 0)
            UF.unite(i, P[i]);

    vector<int> regular, special;
    vector<bool> done(N, false);

    for (int i = 0; i < N; i++)
        if (P[i] < 0) {
            int root = UF.find(i);
            special.push_back(UF.get_size(root));
            done[root] = true;
        }

    for (int i = 0; i < N; i++)
        if (!done[i] && UF.find(i) == i)
            regular.push_back(UF.get_size(i));

    int R = int(regular.size());
    int S = int(special.size());
    int special_sum = 0;

    for (int s : special)
        special_sum += s;

    mod_int answer = mod_int(N) * mod_int(N - 1).pow(S);
    dbg(answer);
    mod_int component_total = (R + S) * mod_int(N - 1).pow(S);
    dbg(component_total);

    for (int s : special)
        component_total -= (N - s) * mod_int(N - 1).pow(S - 1);

    dbg(component_total);
    vector<mod_int> dp(S + 1, 0);
    dp[0] = mod_int(N - 1).pow(S);
    mod_int cycle_total = 0;

    for (int s = 0; s < S; s++) {
        vector<mod_int> next_dp(S + 1, 0);

        for (int c = 0; c <= s; c++)
            next_dp[c + 1] += dp[c] * max(c, 1) / (N - 1) * special[s];

        for (int c = 2; c <= s + 1; c++)
            cycle_total += next_dp[c];

        for (int c = 0; c <= s + 1; c++)
            dp[c] += next_dp[c];
    }

    dbg(cycle_total);
    component_total += cycle_total;
    answer -= component_total;
    cout << answer << '\n';
}


Measured memory_bytes: 3768