#include <bits/stdc++.h>
using namespace std;
const int MAXN = 100005;
const int P = 1e9 + 7;
template <typename T>
void chkmax(T &x, T y) {
  x = max(x, y);
}
template <typename T>
void chkmin(T &x, T y) {
  x = min(x, y);
}
template <typename T>
void read(T &x) {
  x = 0;
  int f = 1;
  char c = getchar();
  for (; !isdigit(c); c = getchar())
    if (c == '-') f = -f;
  for (; isdigit(c); c = getchar()) x = x * 10 + c - '0';
  x *= f;
}
template <typename T>
void write(T x) {
  if (x < 0) x = -x, putchar('-');
  if (x > 9) write(x / 10);
  putchar(x % 10 + '0');
}
template <typename T>
void writeln(T x) {
  write(x);
  puts("");
}
int n, bit[MAXN], cnt[MAXN], a[MAXN];
int main() {
  read(n), bit[0] = 1;
  for (int i = 1; i <= n; i++) {
    bit[i] = bit[i - 1] * 2 % P;
    read(a[i]);
    if (a[i] != -1) cnt[a[i]]++;
  }
  int ans = bit[n - 1] - 1;
  for (int i = 1; i <= n; i++)
    if (a[i] == -1) ans = (ans - (bit[cnt[i]] - 1)) % P;
  writeln((ans + P) % P);
  return 0;
}
