#include <bits/stdc++.h>
#pragma comment(linker, "/STACK:102400000,102400000")
using namespace std;
const int inf = 1000000007;
const long long linf = 1LL << 62;
const unsigned long long ulinf = 1ULL << 63;
const double eps = 0.000001;
const double pi = 3.14159265358979323846;
template <class T>
T abs(T a) {
  return a >= 0 ? a : -a;
}
template <class T>
T sqr(T a) {
  return a * a;
}
template <class T>
T gcd(T a, T b) {
  return b ? gcd(b, a % b) : a;
}
template <class T>
T mod(T a, T b) {
  return (a % b + b) % b;
}
template <class T>
T lowbit(T x) {
  return x & -x;
}
long long addmod(long long a, long long b, long long c) {
  return ((a + b) % c + c) % c;
}
long long powmod(long long a, long long b, long long c) {
  if (b == 0LL) return 1LL;
  long long ret = powmod(a, b >> 1, c);
  ret = ret * ret % c;
  if (b & 1LL) ret = ret * a % c;
  return ret;
}
long long modinv(long long a, long long b) { return powmod(a, b - 2LL, b); }
template <class T>
void maxe(T &a, T b) {
  if (a < b) a = b;
}
template <class T>
void mine(T &a, T b) {
  if (a > b) a = b;
}
int iszero(double a) { return abs(a) <= eps; }
template <class T>
void geti(T &a) {
  a = 0;
  T b = 1;
  char c = getchar();
  if (c == '-')
    b = -1;
  else
    a = c - 48;
  while ((c = getchar()) != ' ' && c != '\n') a = a * 10 + c - 48;
  a *= b;
}
void fileio_in_out() {
  freopen(".in", "r", stdin);
  freopen(".out", "w", stdout);
}
void fileio_txt() {
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
}
const int N = 1111111;
const int M = 1111111;
const int K = 22;
int test;
int n, m, k;
long long ans;
const long long md = (long long)inf;
int id[N], pos[N];
long long fac[N];
int l, r, cnt;
long long cmn(int m, int n) {
  return fac[m] * modinv(fac[n], md) % md * modinv(fac[m - n], md) % md;
}
long long getans() {
  long long ret = 1;
  l = 1;
  r = n;
  cnt = 0;
  for (int i = n; i; i--) {
    cnt++;
    if (pos[i]) {
      if (pos[i] < l || pos[i] > r) return 0;
      long long t;
      if (i == k) {
        if (i == 1) {
          t = cmn(cnt - 1, pos[i] - l);
        } else {
          t = 0;
          if (pos[i] + (i - 1) <= r) t = (t + cmn(cnt - 1, pos[i] - l)) % md;
          if (pos[i] - (i - 1) >= l) t = (t + cmn(cnt - 1, r - pos[i])) % md;
        }
      } else if (pos[i] < pos[k]) {
        t = cmn(cnt - 1, pos[i] - l);
        r -= cnt - (pos[i] - l + 1);
        l = pos[i] + 1;
      } else {
        t = cmn(cnt - 1, r - pos[i]);
        l += cnt - (r - pos[i] + 1);
        r = pos[i] - 1;
      }
      ret = ret * t % md;
      cnt = 0;
    }
  }
  if (cnt) ret = ret * powmod(2LL, cnt - 1, md) % md;
  return ret;
}
int main() {
  scanf("%d", &n);
  k = inf;
  fac[0] = 1;
  for (int i = 1; i <= n; i++) {
    scanf("%d", &id[i]);
    if (id[i]) {
      pos[id[i]] = i;
      mine(k, id[i]);
    }
    fac[i] = fac[i - 1] * i % md;
  }
  ans = getans();
  printf("%I64d\n", ans);
  return 0;
}
