#include <algorithm>
#include <cassert>
#include <cfloat>
#include <climits>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <deque>
#include <iostream>
#include <iterator>
#include <limits>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

#define FOR(i,k,n) for(int(i)=(k);(i)<(n);++(i))
#define rep(i,n) FOR(i,0,n)
#define all(v) begin(v),end(v)
#ifdef ONLINE_JUDGE
  #define debug(x)
  #define debug2(x,y)
  #define debug3(x,y,z)
  #define assert(e) if(!(e)){stod("");}
#else
  #define debug(x) std::cerr<<#x<<": "<<x<<"\n"
  #define debug2(x,y) std::cerr<<#x<<": "<<x<<", "<<#y<<": "<<y<<"\n"
  #define debug3(x,y,z) std::cerr<<#x<<": "<<x<<", "<<#y<<": "<<y<<", "<<#z<<": "<<z<<"\n"
#endif

using ll=long long;
using vi=std::vector<int>;
using vvi=std::vector<vi>;
using vll=std::vector<ll>;
using vvll=std::vector<vll>;
template<typename T> using vvec=std::vector<std::vector<T>>;
template<typename T>
auto make_v(size_t sz){return std::vector<T>(sz);}
template<typename T,typename... Ts>
auto make_v(size_t sz,Ts...ts){return std::vector<decltype(make_v<T>(ts...))>(sz,make_v<T>(ts...));}
template<typename T>
void fill_v(T&var,const T&x){var=x;}
template<typename V,typename T>
void fill_v(V&v,const T&x){for(auto&& w:v){fill_v(w,x);}}

template<typename T> std::ostream& operator<<(std::ostream&s,const std::vector<T>&v){
  int sz=v.size();s<<"\n";rep(i,sz){s<<v[i];if(i<sz-1){s<<"\t";}}s<<"\n";return s;}
template<typename T> std::ostream& operator<<(std::ostream&s,const std::vector<std::vector<T>>&v){
  for(auto&& w:v){s<<w;}return s;}
template<typename T> std::ostream& operator<<(std::ostream&s,const std::deque<T>&v){
  int sz=v.size();s<<"\n";rep(i,sz){s<<v[i];if(i<sz-1){s<<"\t";}}s<<"\n";return s;}
template<typename T> std::ostream& operator<<(std::ostream&s,const std::deque<std::deque<T>>&v){
  for(auto&& w:v){s<<w;}return s;}
template<typename T> std::ostream& operator<<(std::ostream&s, const std::set<T>&v){
  s<<"\n";for(auto&& elm:v){s<<elm<<"\t";}s<<"\n";return s;}

inline void scan(int&a){scanf("%d",&a);}
inline void scan(ll&a){scanf("%lld",&a);}
inline void scan(char&a){scanf(" %c",&a);}
inline void scan(double&a){scanf("%lf",&a);}
template<typename T>
inline void scan(std::vector<T>&v){for(auto&& sv:v){scan(sv);}}
template<typename First,typename...Args>
inline void scan(First&f,Args&...args){scan(f);scan(args...);}
inline void scan(std::string&s){char BUF[3000000];scanf(" %s",BUF);s=std::string(BUF);}
inline void print(int a){printf("%d\n",a);}
inline void print(ll a){printf("%lld\n",a);}
inline void print(double a){printf("%.12f\n",a);}
inline void print(std::string s){std::cout<<s<<"\n";}

using namespace std;

class UF {
private:
  vector<int> data; // parent or size
  vector<int> next;
  vector<int> last;
  void init(int n) {
    data.assign(n, -1);
    next.assign(n, -1);
    last.resize(n);
    for (int i = 0; i < n; ++i) {
      last[i] = i;
    }
  }

public:
  UF() {}
  UF(int n) {
    init(n);
  }
  int root(int x) {
    if (data[x] < 0) return x;
    return data[x] = root(data[x]);
  }
  bool unite(int x, int y) {
    x = root(x);
    y = root(y);
    if (x == y) return false;
    if (data[x] > data[y]) swap(x, y); // data[x] and data[y] are negative.
    data[x] += data[y];
    data[y] = x;
    next[last[x]] = y;
    last[x] = last[y];
    return true;
  }
  int size(int x) {
    return -data[root(x)];
  }
  bool same(int x, int y) {
    return root(x) == root(y);
  }
  int get_next(int x) {
    return next[x];
  }
};

vvi adj;
vi repr;
void dfs(int u, int r) {
  repr.at(u) = r;
  for (int v : adj.at(u)) {
    if (repr.at(v) == -1) {
      dfs(v, r);
    }
  }
}

int mod = 1000000007;
ll po(ll k, ll x) {
  if (x == 0) {
    return 1;
  }
  if (x == 1) {
    return k % mod;
  }
  ll y = po(k, x/2);
  y = y * y % mod;
  if (x % 2 == 1) {
    y = y * k % mod;
  }
  return y;
}

void solve() {
  int n;
  scan(n);
  vi p(n);
  scan(p);
  adj.resize(n);
  UF uf(n);
  rep (i, n) {
    if (p.at(i) != -1) {
      p.at(i) -= 1;
      adj.at(i).push_back(p.at(i));
      adj.at(p.at(i)).push_back(i);
      uf.unite(i, p.at(i));
    }
  }
  repr.assign(n, -1);
  int isolated = 0;
  vi ncic, cic;
  rep (i, n) {
    if (i != uf.root(i)) {
      continue;
    }
    if (uf.size(i) == 1) {
      isolated += 1;
    }
    int u = i;
    bool ncic_flg = false;
    while (u != -1) {
      if (p[u] == -1) {
        ncic_flg = true;
        break;
      }
      u = uf.get_next(u);
    }
    if (ncic_flg) {
      ncic.push_back(uf.size(i));
    } else {
      cic.push_back(uf.size(i));
    }
  }
  debug3(ncic, cic, isolated);
  int m = 10000;
  vll frac(m, 0);
  vll rfrac(m, 0);
  frac[0] = rfrac[0] = 1;
  rep (i, m-1) {
    frac[i+1] = frac[i] * (i+1) % mod;
    rfrac.at(i+1) = po(frac.at(i+1), mod-2);
  }
  sort(all(ncic));
  int sz = ncic.size();
  if (sz == 0) {
    ll ans = n - cic.size();
    print(ans);
    return;
  }
  // Calculate the total number of strongly connected components. (tnscc)
  vvll dp(sz+1, vll(sz+1, 0));
  dp.at(0).at(0) = 1;
  rep (i, sz) {
    dp.at(i+1).at(0) = 1;
    rep (j, i+1) {
      dp.at(i+1).at(j+1) = (dp.at(i).at(j) * ncic.at(i) + dp.at(i).at(j+1)) % mod;;
    }
  }
  debug(dp);
  ll tnscc = 0;
  tnscc = (dp.at(sz).at(1) - sz) * po(n-1, sz-1) % mod;
  debug(tnscc);
  FOR (i, 2, sz+1) {
    tnscc += dp.at(sz).at(i) * frac.at(i-1) % mod * po(n-1, sz-i);
    tnscc %= mod;
    debug(tnscc);
  }
  tnscc = (tnscc + po(n-1, sz) * cic.size()) % mod;
  debug(tnscc);
  ll ans = (po(n-1, sz) * n + (mod - tnscc)) % mod;
  print(ans);
}

int main() {
  solve();
  return 0;
}


Measured memory_bytes: 199040