#include <bits/stdc++.h>

using i32 = int_fast32_t;
using i64 = int_fast64_t;
using u32 = uint_fast32_t;
using u64 = uint_fast64_t;
using f64 = double;
using f80 = long double;

#define FOR(v, a, b) for(i64 v = (a); v < (b); ++v)
#define FORE(v, a, b) for(i64 v = (a); v <= (b); ++v)
#define REP(v, n) FOR(v, 0, n)
#define REPE(v, n) FORE(v, 0, n)
#define REV(v, a, b) for(i64 v = (a); v >= (b); --v)
#define ALL(x) (x).begin(), (x).end()
#define RALL(x) (x).rbegin(), (x).rend()
#define ITR(it, c) for(auto it = (c).begin(); it != (c).end(); ++it)
#define RITR(it, c) for(auto it = (c).rbegin(); it != (c).rend(); ++it)
#define EXIST(c,x) ((c).find(x) != (c).end())
#define fst first
#define snd second
#define UNIQ(v) (v).erase(unique(ALL(v)), (v).end())
#define bit(i) (1LL<<(i))

#ifdef DEBUG
#include <Mylib/Debug/debug.cpp>
#else
#define dump(...) ((void)0)
#endif

using namespace std;

template <typename I> void join(ostream &ost, I s, I t, string d=" "){for(auto i=s; i!=t; ++i){if(i!=s)ost<<d; ost<<*i;}ost<<endl;}
template <typename T> istream& operator>>(istream &is, vector<T> &v){for(auto &a : v) is >> a; return is;}

template <typename T, typename U> bool chmin(T &a, const U &b){return (a>b ? a=b, true : false);}
template <typename T, typename U> bool chmax(T &a, const U &b){return (a<b ? a=b, true : false);}

template <typename T, size_t N, typename U> void fill_array(T (&a)[N], const U &v){fill((U*)a, (U*)(a+N), v);}
template <typename T> auto make_vector(int n, int m, const T &value){return vector<vector<T>>(n, vector<T>(m, value));}

template <typename T> void print(const T &value){std::cout << value << "\n";}
template <typename T, typename ...Args> void print(const T &value, const Args&... args){std::cout << value << " ";print(args...);}


struct Init{
  Init(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(12);
    cerr << fixed << setprecision(12);
  }
}init;



/**
 * @title modint
 * @docs mint.md
 */
template <uint32_t M> class ModInt{
public:
  constexpr static uint32_t MOD = M;
  uint64_t val;
  
  constexpr ModInt(): val(0){}
  constexpr ModInt(int64_t n){
    if(n >= M) val = n % M;
    else if(n < 0) val = n % M + M;
    else val = n;
  }
  
  inline constexpr auto operator+(const ModInt &a) const {return ModInt(val + a.val);}
  inline constexpr auto operator-(const ModInt &a) const {return ModInt(val - a.val);}
  inline constexpr auto operator*(const ModInt &a) const {return ModInt(val * a.val);}
  inline constexpr auto operator/(const ModInt &a) const {return ModInt(val * a.inv().val);}
  
  inline constexpr auto& operator=(const ModInt &a){val = a.val; return *this;}
  inline constexpr auto& operator+=(const ModInt &a){if((val += a.val) >= M) val -= M; return *this;}
  inline constexpr auto& operator-=(const ModInt &a){if(val < a.val) val += M; val -= a.val; return *this;}
  inline constexpr auto& operator*=(const ModInt &a){(val *= a.val) %= M; return *this;}
  inline constexpr auto& operator/=(const ModInt &a){(val *= a.inv().val) %= M; return *this;}
  
  inline constexpr bool operator==(const ModInt &a) const {return val == a.val;}
  inline constexpr bool operator!=(const ModInt &a) const {return val != a.val;}
  
  inline constexpr auto& operator++(){*this += 1; return *this;}
  inline constexpr auto& operator--(){*this -= 1; return *this;}
  
  inline constexpr auto operator++(int){auto t = *this; *this += 1; return t;}
  inline constexpr auto operator--(int){auto t = *this; *this -= 1; return t;}
  
  inline constexpr static ModInt power(int64_t n, int64_t p){
    if(p < 0) return power(n, -p).inv();
    
    int64_t ret = 1, e = n % M;
    for(; p; (e *= e) %= M, p >>= 1) if(p & 1) (ret *= e) %= M;
    return ret;
  }
  
  inline constexpr static ModInt inv(int64_t a){
    int64_t b = M, u = 1, v = 0;
    
    while(b){
      int64_t t = a / b;
      a -= t * b; std::swap(a,b);
      u -= t * v; std::swap(u,v);
    }
    
    u %= M;
    if(u < 0) u += M;
    
    return u;
  }
  
  inline constexpr static auto frac(int64_t a, int64_t b){return ModInt(a) / ModInt(b);}
  
  inline constexpr auto power(int64_t p) const {return power(val, p);}
  inline constexpr auto inv() const {return inv(val);}
  
  friend inline constexpr auto operator-(const ModInt &a){return ModInt(-a.val);}
  
  friend inline constexpr auto operator+(int64_t a, const ModInt &b){return ModInt(a) + b;}
  friend inline constexpr auto operator-(int64_t a, const ModInt &b){return ModInt(a) - b;}
  friend inline constexpr auto operator*(int64_t a, const ModInt &b){return ModInt(a) * b;}
  friend inline constexpr auto operator/(int64_t a, const ModInt &b){return ModInt(a) / b;}
  
  friend std::istream& operator>>(std::istream &s, ModInt<M> &a){s >> a.val; return s;}
  friend std::ostream& operator<<(std::ostream &s, const ModInt<M> &a){s << a.val; return s;}

  template <int N>
  inline static auto div(){
    static auto value = inv(N);
    return value;
  }

  explicit operator int32_t() const noexcept {return val;}
  explicit operator int64_t() const noexcept {return val;}
};

using mint = ModInt<1000000007>;


/**
 * @title UnionFind
 * @docs unionfind.md
 */
class UnionFind{
  std::vector<int> parent, depth, size;
  int count;

public:
  UnionFind(int n): parent(n), depth(n,1), size(n,1), count(n){
    std::iota(parent.begin(), parent.end(), 0);
  }
  
  inline int get_root(int i){
    if(parent[i] == i) return i;
    else return parent[i] = get_root(parent[i]);
  }
  
  inline bool is_same(int i, int j){return get_root(i) == get_root(j);}

  inline int merge(int i, int j){
    int ri = get_root(i), rj = get_root(j);
    if(ri == rj) return ri;
    else{
      --count;
      if(depth[ri] < depth[rj]){
        parent[ri] = rj;
        size[rj] += size[ri];
        return rj;
      }else{
        parent[rj] = ri;
        size[ri] += size[rj];
        if(depth[ri] == depth[rj]) ++depth[ri];
        return ri;
      }
    }
  }

  inline int get_size(int i){return size[get_root(i)];}

  inline int count_group(){return count;}
};

mint factorial(int K){
  mint ret = 1;
  FORE(i,1,K) ret *= i;
  return ret;
}


int main(){
  int N;

  while(cin >> N){
    vector<int> P(N); cin >> P;

    int K = 0;
    UnionFind uf(N);

    REP(i,N){
      if(P[i] == -1) ++K;
      else{
        P[i] -= 1;
        uf.merge(i, P[i]);
      }
    }

    int C = uf.count_group();
    

    vector<int> S;
    REP(i,N){
      if(P[i] == -1) S.push_back(uf.get_size(i));
    }

    auto dp = make_vector<mint>(K+1, K+1, 0);

    dp[0][0] = 1;

    REP(i,K){
      REPE(j,K){
        if(j + 1 <= K) dp[i+1][j+1] += dp[i][j] * S[i];
        dp[i+1][j] += dp[i][j];
      }
    }

    mint t = 0;

    REP(i,K){
      t += (S[i] - 1) * mint::power(N-1, K-1);
    }

    FORE(i,2,K){
      t += dp[K][i] * factorial(i-1) * mint::power(N-1, K-i);
    }

    auto ans = mint::power(N-1, K) * (N - (C - K)) - t;
    
    print(ans);
  }

  return 0;
}


Measured memory_bytes: 198780