#line 1 "main_d.cpp"
#include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <cctype>
#include <cstdint>
#include <cstdlib>
#include <cmath>
#include <complex>
#include <chrono>
#include <deque>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <memory>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <type_traits>
#include <unordered_map>
#include <vector>
#include <random>
#include <utility>
#include <limits>
#include <list>

/* template start */
 
#define rep(i, a, b) for (long long i = (a); (i) < (b); (i)++)
#define all(i) i.begin(), i.end()

#ifdef LOCAL
#define debug(...) std::cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
#define debug(...)
#endif

void debug_out(){std::cerr<<std::endl;}

template<typename Head,typename... Tail>
void debug_out(Head h,Tail... t){
  std::cerr<<" "<<h;
  if(sizeof...(t)>0)std::cout<<" :";
  debug_out(t...);
}
 
template <typename T1, typename T2>
std::ostream& operator<<(std::ostream& os, std::pair<T1, T2> pa) {
  return os << pa.first << " " << pa.second;
}
 
template <typename T>
std::ostream& operator<<(std::ostream& os, std::vector<T> vec) {
  for (std::size_t i = 0; i < vec.size(); i++)os << vec[i] << (i + 1 == vec.size() ? "" : " ");
  return os;
}
 
template<typename T1,typename T2>
inline bool chmax(T1& a,T2 b){return a<b && (a=b,true);}
 
template<typename T1,typename T2>
inline bool chmin(T1& a,T2 b){return a>b && (a=b,true);}

template<typename Num>
constexpr Num mypow(Num a, unsigned long long b) {
  if(b==0)return 1;
  if (a==0)return 0;
  Num x = 1;
  while (b > 0) {
    if(b & 1)x*=a;
    a*=a;
    b >>= 1;
  }
  return x;
}

/* template end */

using ll = long long;

#line 2 "/home/imperi/Documents/cp-library/lib/local/modint.cpp"

template <std::uint_fast64_t Modulus> class modint {
  using u64 = std::uint_fast64_t;

public:
  u64 a;

  constexpr modint(const u64 x = 0) noexcept : a(x % Modulus) {}
  constexpr u64 &value() noexcept { return a; }
  constexpr const u64 &value() const noexcept { return a; }
  constexpr bool operator==(const modint rhs) const noexcept {return a==rhs.a;}
  constexpr bool operator!=(const modint rhs) const noexcept {return !(*this==rhs);}
  constexpr modint operator+(const modint rhs) const noexcept {
    return modint(*this) += rhs;
  }
  constexpr modint operator-(const modint rhs) const noexcept {
    return modint(*this) -= rhs;
  }
  constexpr modint operator*(const modint rhs) const noexcept {
    return modint(*this) *= rhs;
  }
  constexpr modint operator/(const modint rhs) const noexcept {
    return modint(*this) /= rhs;
  }
  constexpr modint &operator+=(const modint rhs) noexcept {
    a += rhs.a;
    if (a >= Modulus) {
      a -= Modulus;
    }
    return *this;
  }
  constexpr modint &operator-=(const modint rhs) noexcept {
    if (a < rhs.a) {
      a += Modulus;
    }
    a -= rhs.a;
    return *this;
  }
  constexpr modint &operator*=(const modint rhs) noexcept {
    a = a * rhs.a % Modulus;
    return *this;
  }
  constexpr modint &operator/=(modint rhs) noexcept {
    u64 exp = Modulus - 2;
    while (exp) {
      if (exp % 2) {
        *this *= rhs;
      }
      rhs *= rhs;
      exp /= 2;
    }
    return *this;
  }
};
#line 2 "/home/imperi/Documents/cp-library/lib/UnionFind/UnionFind.cpp"

#include <cstddef>
#line 6 "/home/imperi/Documents/cp-library/lib/UnionFind/UnionFind.cpp"

class UnionFind{
  public:
  using size_t=std::size_t;
  private:
  std::vector<size_t> par,sz;
  size_t group;

  public:

  UnionFind(size_t n=0):par(n,0),sz(n,1),group(n){
    std::iota(par.begin(),par.end(),0);
  }

  size_t root(size_t n){
    if(par[n]==n)return n;
    return par[n]=root(par[n]);
  }

  bool unite(size_t a,size_t b){
    a=root(a);b=root(b);
    if(a==b)return false;
    group--;
    if(sz[a]<sz[b])std::swap(a,b);
    sz[a]+=sz[b];
    par[b]=a;
    return true;
  }

  size_t size(){return par.size();}
  size_t groups(){return group;}
  size_t group_size(size_t n){return sz[root(n)];}

  bool same(size_t a,size_t b){return root(a)==root(b);}
};
#line 86 "main_d.cpp"

constexpr ll MOD=1e9+7;
using mint=modint<MOD>;

int main() {
  std::cin.tie(nullptr);
  std::ios::sync_with_stdio(false);

  ll n;
  std::cin>>n;

  ll cnt=0;

  UnionFind uni(n);

  std::vector<ll> p(n);
  std::vector<std::vector<ll>> graph(n);
  for(auto&& e:p){
    std::cin>>e;
    if(e>0)e--;
    else cnt++;
  }

  rep(i,0,n){
    if(p[i]!=-1){
      graph[i].emplace_back(p[i]);
      graph[p[i]].emplace_back(i);
    }
  }

  using P=std::pair<ll,ll>;

  std::vector<bool> visit(n,0);

  auto dfs=[&](auto f,ll now,ll par)->P{
    ll size=1;
    bool ret=(p[now]==-1);
    visit[now]=true;
    for(const auto& e:graph[now]){
      if(e==par||e==now)continue;
      if(visit[e])continue;
      P tmp=f(f,e,now);
      size+=tmp.first;
      ret+=tmp.second;
    }
    return {size,ret};
  };

  std::vector<P> compo;

  rep(i,0,n){
    if(!visit[i])compo.emplace_back(dfs(dfs,i,-1));
  }

  std::sort(all(compo),[](P a,P b){return a.second<b.second;});

  mint ans=0;

  ll m=compo.size();

  rep(i,0,m)ans+=compo[i].first-1;

  ans*=mypow(mint(n-1),cnt);

  debug(ans.value());

  for(const auto& e:compo){
    if(e.second){
      ans+=mypow(mint(n-1),cnt-1)*(n-e.first);
    }
  }

  debug(ans.value());

  std::vector<mint> cycle(cnt+1,0);
  cycle[0]=1;

  for(const auto& e:compo){
    if(e.second){
      std::vector<mint> tmp(cnt+1,0);
      rep(i,0,cnt+1)tmp[i]+=cycle[i];
      rep(i,1,cnt+1)tmp[i]+=cycle[i-1]*e.first;
      cycle=tmp;
    }
  }

  rep(i,0,cnt+1)debug(cycle[i].value());

  std::vector<mint> perm(cnt+1,1);
  rep(i,1,cnt+1)perm[i]=perm[i-1]*i;

  rep(i,2,cnt+1){
    ans-=cycle[i]*perm[i-1]*mypow(mint(n-1),cnt-i);
  }

  std::cout<<ans.value()<<"\n";

  return 0;
}


Measured memory_bytes: 4100