#include <bits/stdc++.h>
#include <tr1/unordered_map>
#include <tr1/unordered_set>
#define ri register int
#define ptf printf
using namespace std;
typedef double db;
typedef long double ld;
typedef long long ll;
typedef vector <int> poly;
typedef pair <int, int> pii;
typedef pair <ll, int> pli;
typedef pair <int, ll> pil;
typedef pair <ll, ll> pll;
typedef unsigned long long ulll;
typedef unsigned int uii;
typedef string strr;
#define fi first
#define se second
#define pb push_back
#define ppp pop_back
#define rez resize
const ll Inf = 2e18;
const int rlen = 1 << 20, inf = 1e9;
char buf[rlen], *ib = buf, *ob = buf;
#define gc() (((ib == ob) && (ob = (ib =  buf) + fread(buf, 1, rlen, stdin)), ib == ob) ? -1 : *ib++)
inline int read() {
  static int ans, f;
  static char ch;
  ans = 0, ch = gc(), f = 1;
  while (!isdigit(ch)) f ^= ch == '-', ch = gc();
  while (isdigit(ch)) ans = (ans << 3) + (ans << 1) + (ch ^ 48), ch = gc();
  return f ? ans: -ans;
}
inline ll readl() {
  static ll ans;
  static char ch;
  for (ans = 0, ch = gc(); !isdigit(ch); ch = gc());
  while (isdigit(ch)) ans = ((ans << 2) + ans << 1) + (ch ^ 48), ch = gc();
  return ans;
}
inline int Read(char *s) {
  static int top;
  static char ch;
  top = 0, ch = gc();
  while (!isalpha(ch) && !isdigit(ch)) ch = gc();
  while (isalpha(ch) || isdigit(ch)) s[++top] = ch, ch = gc();
  return top;
}
namespace modular {
  const int mod = 1e9 + 7;// = 998244353;
  //int mod;
  int ret;
  inline int add(int a, int b) { return a + b < mod ? a + b : a + b - mod; }
  inline int dec(int a, int b) { return a < b ? a - b + mod : a - b; }
  inline int mul(int a, int b) { return (ll) a * b % mod; }
  inline void Add(int &a, int b) { a = a + b < mod ? a + b : a + b - mod; }
  inline void Dec(int &a, int b) { a = a < b? a - b + mod : a - b; }
  inline void Mul(int &a, int b) { a = (ll) a * b % mod; }
  inline int ksm(int a, int p) {
    for (ret = 1; p; p >>= 1, Mul(a, a)) (p & 1) && (Mul(ret, a), 1);
    return ret;
  }
  inline int Inv(int a) { return ksm(a, mod - 2); }
  inline int sqr(int a) { return (ll) a * a % mod; }
  inline int cub(int a) { return (ll) a * a % mod * a % mod; }
} using namespace modular;
template <typename T> inline void ckmax(T &a, T b) { a < b ? a = b : 0; }
template <typename T> inline void ckmin(T &a, T b) { a > b ? a = b : 0; }
template <typename T> inline T Abs(T a) { return a < 0 ? -a : a; }
template <typename T> inline T gcd(T a, T b) {
  T t;
  while (b) t = a, a = b, b = t - t / a * a;
  return a;
}
template <typename T> inline void exgcd(T a, T b, T &x, T &y) {
  if (!b) {
    x = 1, y = 0;
    return;
  }
  exgcd(b, a - a / b * b, y, x);
  y -= a / b * x;
}
const int N = 5005;
int n, a[N], fac[N], sz, pw[N], m = 0;
bool vs[N];
vector <int> e[N];
void dfs(int p) {
  ++sz, vs[p] = 1;
  for (ri i = 0; i < e[p].size(); ++i) dfs(e[p][i]); 
}
void Dfs(int p, int anc) {
  vs[p] = 1;
  for (ri i = 0, v; i < e[p].size(); ++i) {
    if (!vs[v = e[p][i]]) Dfs(v, anc);
    else if (v == anc) ++m;
  }
}
inline poly operator * (poly a, poly b) {
  int n = a.size(), m = b.size(), t = n + m - 1;
  poly c(t);
  for (ri i = 0; i < n; ++i) if (a[i]) for (ri j = 0; j < m; ++j) Add(c[i + j], mul(a[i], b[j]));
  return c;
}
int main() {
  #ifdef ldxcaicai
  freopen("lx.in", "r", stdin);
  #endif
  n = read();
  fac[0] = pw[0] = 1;
  for (ri i = 1; i <= n; ++i) {
    a[i] = read(), fac[i] = mul(fac[i - 1], i), pw[i] = mul(pw[i - 1], n - 1);
    if (~a[i]) e[a[i]].pb(i);
  }
  poly res(1, 1), tp;
  int dt = 0;
  for (ri i = 1; i <= n; ++i) if (!(~a[i])) sz = 0, dfs(i), tp = poly(2, 1), tp[1] = sz, res = res * tp, ++dt;
  for (ri i = 1; i <= n; ++i) if (!vs[i]) Dfs(i, i);
  if (res.size() > 1) Dec(res[1], dt);
  int ss = mul(n - m, pw[dt]);
  for (ri i = 1; i <= dt; ++i) Dec(ss, mul(res[i], mul(fac[i - 1], pw[dt - i])));
  cout << ss;
  return 0;
}

Measured memory_bytes: 4028