/**
 * code generated by JHelper
 * More info: https://github.com/AlexeyDmitriev/JHelper
 * @author aajisaka
 */

#include<bits/stdc++.h>

using namespace std;

void debug_out() { cerr << endl; }
template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
  cerr << " " << to_string(H);
  debug_out(T...);
}
#ifdef LOCAL
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
#define debug(...) 42
#endif

#define SPEED ios_base::sync_with_stdio(false);cin.tie(nullptr)
#define rep(i,n) for(int i=0; i<(int)(n); i++)
#define all(v) v.begin(), v.end()
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return true; } return false; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return true; } return false; }

using ll = long long;
using ull = unsigned long long;
using P = pair<ll, ll>;

constexpr long double PI = 3.14159265358979323846264338327950288L;
mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
constexpr ll mod17 = 1e9+7;
constexpr ll mod19 = 1e9+9;
constexpr ll mod9 = 998244353;
ll mod = mod17;

// Mod int libraries
template<typename T>
T mod_pow(T a, ll x) {
  T res = 1;
  while(x > 0) {
    if (x & 1) res *= a;
    a *= a; x >>= 1;
  }
  return res;
}

// Mint32
unordered_map<ll, ll> minvmap;
ll minv(ll a, ll m) {
  auto k = a; auto p = minvmap[a]; if (p != 0) return p;
  ll b = m, u = 1, v = 0;
  while (b) { ll t = a/b; swap(a -= t*b, b); swap(u -= t*v, v); }
  p = (u%m+m)%m; minvmap[k] = p; return p;
}

struct mint {
  ll x;
  mint():x(0){}
  mint(ll x):x((x%mod+mod)%mod){}
  mint& fix() { x = (x%mod+mod)%mod; return *this;}
  mint operator-() const { return mint(0) - *this;}
  mint& operator+=(const mint& a){ if((x+=a.x)>=mod) x-=mod; return *this;}
  mint& operator-=(const mint& a){ if((x+=mod-a.x)>=mod) x-=mod; return *this;}
  mint& operator*=(const mint& a){ (x*=a.x)%=mod; return *this;}
  mint& operator/=(const mint& a){ (x*=minv(a.x, mod))%=mod; return *this; }
  mint operator+(const mint& a)const{ return mint(*this) += a;}
  mint operator-(const mint& a)const{ return mint(*this) -= a;}
  mint operator*(const mint& a)const{ return mint(*this) *= a;}
  mint operator/(const mint& a)const{ return mint(*this) /= a;}
  bool operator<(const mint& a)const{ return x < a.x;}
  bool operator==(const mint& a)const{ return x == a.x;}
};

// Mint64
struct mint64 {
    ull x;
    static ull mod, inv, r2;
    mint64(): x(0){}
    mint64(ull x): x(init(x)) {}
    static ull init(ull x) { return reduce(__uint128_t(x)*r2);}
    static void set_mod(ull m) {
      mod = inv = m;
      for(int i=0; i<5; i++) inv *= 2 - inv*m;
      r2 = -__uint128_t(m)%m;
    }
    static ull reduce(__uint128_t x) {
      ull y = ull(x >> 64) - ull((__uint128_t(ull(x)*inv)*mod)>>64);
      return ll(y) < 0 ? y+mod : y;
    }
    mint64& operator+=(mint64& a) { x+=a.x-mod; if(ll(x)<0) x+=mod; return *this; }
    mint64 operator+(mint64& a) const{ return mint64(*this)+=a; }
    mint64& operator*=(mint64& a) { x=reduce(__uint128_t(x)*a.x); return *this; }
    mint64 operator*(mint64& a) const { return mint64(*this)*=a; }
};

ull mint64::mod, mint64::inv, mint64::r2;

// Verify: https://yukicoder.me/submissions/472580
struct Miller {
    const vector<ull> v = {2, 325, 9375, 28178, 450775, 9780504, 1795265022};
    bool suspect(ull a, ull s, ull d, ull n) {
      if (mint64::mod != n) mint64::set_mod(n);
      mint64 one = 1, minusone = n-1, ma = a;
      auto x = mod_pow(ma, d);
      if (x.x == one.x) return true;
      for(int r = 0; r < s; r++) {
        if (x.x == minusone.x) return true;
        x = x * x;
      }
      return false;
    }

    // check if n is prime
    bool check(ull n) {
      if (n < 2 || (n > 2 && n % 2 == 0)) return false;
      ull d = n - 1;
      ull s = 0;
      while (!(d & 1)) {
        d >>= 1;
        s++;
      }
      for (auto a: v) {
        if (a >= n) break;
        if (!suspect(a, s, d, n)) return false;
      }
      return true;
    }
};

class combination {
    // factorial
    public: std::vector<mint> fact;
    public: std::vector<mint> inv;

    combination(int n) {
      fact.resize(n + 1);
      inv.resize(n + 1);
      fact[0] = 1;
      for (int i = 1; i <= n; i++) {
        fact[i] = fact[i - 1] * i;
      }
      inv[n] = mint(1) / fact[n];
      for (int i = n - 1; i >= 0; i--) {
        inv[i] = inv[i + 1] * (i + 1);
      }
    }

    // nCr
    public: mint get(int n, int r) {
      if (n < r || n < 0 || r < 0) return 0;
      return fact[n]*inv[r]*inv[n-r];
    }
    // nPr
    public: mint p(int n, int r) {
      if (n < r || n < 0) return 0;
      return fact[n]*inv[n-r];
    }
};


class DUrbanPlanning {
public:

    std::vector<int> par;
    std::vector<int> rak; // number of the vertices

    void init(int n) {
      par.resize(n);
      rak.resize(n);
      for(int i=0; i<n; i++) {
        par[i] = i;
        rak[i] = 1;
      }
    }

    int find(int x) {
      if (par[x] == x) {
        return x;
      } else {
        return par[x] = find(par[x]);
      }
    }

    void unite(int x, int y) {
      x = find(x);
      y = find(y);
      if (x==y) return;
      if (rak[x] < rak[y]) {
        rak[y] += rak[x];
        par[x] = y;
      } else {
        rak[x] += rak[y];
        par[y] = x;
      }
    }

    bool same(int x, int y) {
      return find(x) == find(y);
    }

    void solve(istream& cin, ostream& cout) {
      SPEED;
      int n; cin >> n;
      vector<int> p(n);
      init(n);
      mint ans1 = 0;
      rep(i, n) {
        cin >> p[i];
        if (p[i] != -1) {
          p[i]--;
          if (!same(i, p[i])) {
            ans1 += 1;
            unite(i, p[i]);
          }
        }
      }

      vector<vector<mint>> dp(n+1, vector<mint>(n+1));
      vector<int> pas;
      for(int i=0; i<n; i++) {
        if (p[i] == -1) {
          int pa = find(i);
          pas.push_back(rak[pa]);
          debug(rak[pa]);
        }
      }

      int si = pas.size();
      dp[0][0] = 1;
      rep(i, si) {
        for(int j=0; j<si; j++) {
          dp[i+1][j+1] += dp[i][j]*pas[i];
          dp[i+1][j] += dp[i][j];
        }
      }
      mint loop = 0;
      mint base = 1;
      for(int i=2; i<=si; i++) {
        //debug(i, dp[si][i].x);
        loop += dp[si][i] * base * mod_pow(mint(n-1), si-i);
        base *= i;
        //debug(loop.x);
      }
      // self-loop
      for(auto e: pas) {
        loop += mod_pow(mint(n-1), si-1) * (e-1);
      }
      //debug(ans1.x);

      mint ans = (ans1+si)*mod_pow(mint(n-1), si);
      ans -= loop;
      debug(loop.x);
      cout << ans.x << endl;




    }
};

signed main() {
  DUrbanPlanning solver;
  std::istream& in(std::cin);
  std::ostream& out(std::cout);
  solver.solve(in, out);
  return 0;
}

Measured memory_bytes: 198792