description: For a permutation p of numbers 1 through n, we define a stair array a as follows: a_i is length of the longest segment of permutation which contains position i and is made of consecutive values in sorted order: [x, x+1, …, y-1, y] or [y, y-1, …, x+1, x] for some x ≤ y. For example, for permutation p = [4, 1, 2, 3, 7, 6, 5] we have a = [1, 3, 3, 3, 3, 3, 3]. 
  
  You are given the stair array a. Your task is to calculate the number of permutations which have stair array equal to a. Since the number can be big, compute it modulo 998 244 353. Note that this number can be equal to zero.
  
  Input
  
  The first line of input contains integer n (1 ≤ n ≤ 10^5) — the length of a stair array a.
  
  The second line of input contains n integers a_1, a_2, …, a_n (1 ≤ a_i ≤ n).
  
  Output
  
  Print the number of permutations which have stair array equal to a. Since the number can be big, compute it modulo 998 244 353.
  
  Examples
  
  Input
  
  
  6
  3 3 3 1 1 1
  
  
  Output
  
  
  6
  
  
  Input
  
  
  7
  4 4 4 4 3 3 3
  
  
  Output
  
  
  6
  
  
  Input
  
  
  1
  1
  
  
  Output
  
  
  1
  
  
  Input
  
  
  8
  2 2 2 2 2 2 1 1
  
  
  Output
  
  
  370
  
  
  Input
  
  
  4
  3 2 3 1
  
  
  Output
  
  
  0
solution: #include <bits/stdc++.h>
  #pragma GCC optimize("O3")
  #pragma GCC optimize("unroll-loops")
  using namespace std;
  constexpr long long mod = 998244353;
  const long long INF = mod * mod;
  const long double eps = 1e-12;
  const long double pi = acosl(-1.0);
  long long mod_pow(long long x, long long n, long long m = mod) {
    if (n < 0) {
      long long res = mod_pow(x, -n, m);
      return mod_pow(res, m - 2, m);
    }
    if (abs(x) >= m) x %= m;
    if (x < 0) x += m;
    long long res = 1;
    while (n) {
      if (n & 1) res = res * x % m;
      x = x * x % m;
      n >>= 1;
    }
    return res;
  }
  struct modint {
    long long n;
    modint() : n(0) { ; }
    modint(long long m) : n(m) {
      if (n >= mod)
        n %= mod;
      else if (n < 0)
        n = (n % mod + mod) % mod;
    }
    operator int() { return n; }
  };
  bool operator==(modint a, modint b) { return a.n == b.n; }
  modint operator+=(modint& a, modint b) {
    a.n += b.n;
    if (a.n >= mod) a.n -= mod;
    return a;
  }
  modint operator-=(modint& a, modint b) {
    a.n -= b.n;
    if (a.n < 0) a.n += mod;
    return a;
  }
  modint operator*=(modint& a, modint b) {
    a.n = ((long long)a.n * b.n) % mod;
    return a;
  }
  modint operator+(modint a, modint b) { return a += b; }
  modint operator-(modint a, modint b) { return a -= b; }
  modint operator*(modint a, modint b) { return a *= b; }
  modint operator^(modint a, long long n) {
    if (n == 0) return modint(1);
    modint res = (a * a) ^ (n / 2);
    if (n % 2) res = res * a;
    return res;
  }
  long long inv(long long a, long long p) {
    return (a == 1 ? 1 : (1 - p * inv(p % a, a)) / a + p);
  }
  modint operator/(modint a, modint b) { return a * modint(inv(b, mod)); }
  modint operator/=(modint& a, modint b) {
    a = a / b;
    return a;
  }
  const int max_n = 1 << 18;
  modint fact[max_n], factinv[max_n];
  void init_f() {
    fact[0] = modint(1);
    for (int i = 0; i < max_n - 1; i++) {
      fact[i + 1] = fact[i] * modint(i + 1);
    }
    factinv[max_n - 1] = modint(1) / fact[max_n - 1];
    for (int i = max_n - 2; i >= 0; i--) {
      factinv[i] = factinv[i + 1] * modint(i + 1);
    }
  }
  modint comb(int a, int b) {
    if (a < 0 || b < 0 || a < b) return 0;
    return fact[a] * factinv[b] * factinv[a - b];
  }
  modint combP(int a, int b) {
    if (a < 0 || b < 0 || a < b) return 0;
    return fact[a] * factinv[a - b];
  }
  int get_premitive_root() {
    int primitive_root = 0;
    if (!primitive_root) {
      primitive_root = [&]() {
        set<int> fac;
        int v = mod - 1;
        for (long long i = 2; i * i <= v; i++)
          while (v % i == 0) fac.insert(i), v /= i;
        if (v > 1) fac.insert(v);
        for (int g = 1; g < mod; g++) {
          bool ok = true;
          for (auto i : fac)
            if (mod_pow(g, (mod - 1) / i) == 1) {
              ok = false;
              break;
            }
          if (ok) return g;
        }
        return -1;
      }();
    }
    return primitive_root;
  }
  const int proot = get_premitive_root();
  void dft(vector<modint>& f, bool inverse = false) {
    int n = f.size();
    if (n == 1) return;
    static vector<modint> w{1}, iw{1};
    for (int m = w.size(); m < n / 2; m *= 2) {
      modint dw = mod_pow(proot, (mod - 1) / (4 * m)), dwinv = (modint)1 / dw;
      w.resize(m * 2);
      iw.resize(m * 2);
      for (int i = 0; i < m; i++) w[m + i] = w[i] * dw, iw[m + i] = iw[i] * dwinv;
    }
    if (!inverse) {
      for (int m = n; m >>= 1;) {
        for (int s = 0, k = 0; s < n; s += 2 * m, k++) {
          for (int i = s; i < s + m; i++) {
            modint x = f[i], y = f[i + m] * w[k];
            f[i] = x + y, f[i + m] = x - y;
          }
        }
      }
    } else {
      for (int m = 1; m < n; m *= 2) {
        for (int s = 0, k = 0; s < n; s += 2 * m, k++) {
          for (int i = s; i < s + m; i++) {
            modint x = f[i], y = f[i + m];
            f[i] = x + y, f[i + m] = (x - y) * iw[k];
          }
        }
      }
      modint n_inv = (modint)1 / (modint)n;
      for (modint& v : f) v *= n_inv;
    }
  }
  vector<modint> multiply(vector<modint> g, vector<modint> h) {
    int n = 1;
    int pi = 0, qi = 0;
    for (int i = 0; i < g.size(); i++)
      if (g[i]) pi = i;
    for (int i = 0; i < h.size(); i++)
      if (h[i]) qi = i;
    int sz = pi + qi + 2;
    while (n < sz) n *= 2;
    g.resize(n);
    h.resize(n);
    dft(g);
    dft(h);
    for (int i = 0; i < n; i++) {
      g[i] *= h[i];
    }
    dft(g, true);
    return g;
  }
  modint inv2 = (1 + mod) / 2;
  struct Data {
    vector<modint> p[2][2];
  };
  Data merge(Data dl, Data dr, int isone) {
    Data res;
    for (int a1 = 0; a1 < 2; a1++)
      for (int b1 = 0; b1 < 2; b1++)
        for (int a2 = 0; a2 < 2; a2++)
          for (int b2 = 0; b2 < 2; b2++) {
            vector<modint> r = multiply(dl.p[a1][b1], dr.p[a2][b2]);
            while (r.size() && r.back() == (modint)0) r.pop_back();
            if (isone && b1 && a2) {
              for (int i = 0; i < r.size(); i++) r[i] *= inv2;
            }
            int na = a1, nb = b2;
            if (res.p[na][nb].size() < r.size()) {
              res.p[na][nb].resize(r.size(), 0);
            }
            for (int i = 0; i < r.size(); i++) {
              res.p[na][nb][i] += r[i];
            }
          }
    return res;
  }
  void solve() {
    int n;
    cin >> n;
    vector<int> a(n);
    for (int i = 0; i < n; i++) cin >> a[i];
    vector<int> c;
    int cur = 0;
    while (cur < n) {
      if (cur + a[cur] > n) {
        cout << 0 << "\n";
        return;
      }
      for (int j = cur; j < cur + a[cur]; j++) {
        if (a[cur] != a[j]) {
          cout << 0 << "\n";
          return;
        }
      }
      c.push_back(a[cur]);
      cur += a[cur];
    }
    if (c.size() == 1) {
      if (n == 1) {
        cout << 1 << "\n";
      } else {
        cout << 2 << "\n";
      }
      return;
    }
    function<Data(int, int)> yaru = [&](int l, int r) -> Data {
      if (r - l == 1) {
        int cnt = 0;
        if (c[l] == 1) cnt++;
        if (c[r] == 1) cnt++;
        Data res;
        res.p[0][0] = {1};
        res.p[1][1] = {0, (1 << cnt)};
        return res;
      }
      int m = (l + r) / 2;
      Data dl = yaru(l, m);
      Data dr = yaru(m, r);
      return merge(dl, dr, c[m] == 1);
    };
    Data d = yaru(0, c.size() - 1);
    vector<modint> num;
    for (int i = 0; i < 2; i++)
      for (int j = 0; j < 2; j++) {
        if (num.size() < d.p[i][j].size()) {
          num.resize(d.p[i][j].size());
        }
        for (int k = 0; k < d.p[i][j].size(); k++) {
          num[k] += d.p[i][j][k];
        }
      }
    int c1 = 0;
    for (int i = 0; i < c.size(); i++)
      if (c[i] == 1) c1++;
    modint coef = mod_pow(inv2, c1);
    modint ans = 0;
    for (int i = 0; i < num.size(); i++) {
      if (i > c.size()) continue;
      modint val = num[i];
      if (i % 2) val *= -1;
      val *= mod_pow(2, c.size() - i);
      val *= fact[c.size() - i];
      val *= coef;
      ans += val;
    }
    cout << ans << "\n";
  }
  signed main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    init_f();
    solve();
    return 0;
  }
  