description: You are given an array of n positive integers a_1, a_2, …, a_n. Your task is to calculate the number of arrays of n positive integers b_1, b_2, …, b_n such that: 
  
    * 1 ≤ b_i ≤ a_i for every i (1 ≤ i ≤ n), and 
    * b_i ≠ b_{i+1} for every i (1 ≤ i ≤ n - 1). 
  
  
  
  The number of such arrays can be very large, so print it modulo 998 244 353.
  
  Input
  
  The first line contains a single integer n (1 ≤ n ≤ 2 ⋅ 10^5) — the length of the array a.
  
  The second line contains n integers a_1, a_2, …, a_n (1 ≤ a_i ≤ 10^9).
  
  Output
  
  Print the answer modulo 998 244 353 in a single line.
  
  Examples
  
  Input
  
  
  3
  2 2 2
  
  
  Output
  
  
  2
  
  Input
  
  
  2
  2 3
  
  
  Output
  
  
  4
  
  Input
  
  
  3
  1 1 1
  
  
  Output
  
  
  0
  
  Note
  
  In the first test case possible arrays are [1, 2, 1] and [2, 1, 2].
  
  In the second test case possible arrays are [1, 2], [1, 3], [2, 1] and [2, 3].
solution: #include <bits/stdc++.h>
  using namespace std;
  constexpr int INF = (numeric_limits<int>::max()) / 2;
  constexpr long long INFLL = (numeric_limits<long long>::max()) / 2;
  template <class T>
  istream &operator>>(istream &is, vector<T> &a) {
    for (auto &i : a) {
      is >> i;
    }
    return is;
  }
  constexpr int mod = 998244353;
  struct segtree {
    struct line {
      int k = -INF, b = -INF;
      line() = default;
      line(int k, int b) : k(k), b(b){};
      bool operator==(line p) const { return k == p.k && b == p.b; }
    };
    struct Node {
      int sum = 0;
      line push;
      Node *l = nullptr, *r = nullptr;
      Node() = default;
    };
    Node *root = nullptr;
    int n = 1e9 + 1;
    void creatend(Node *&v) {
      if (v == nullptr) {
        v = new Node();
      }
    }
    void push(Node *v, int l, int r) {
      if (v->push == line()) {
        return;
      }
      if (l + 1 < r) {
        creatend(v->l);
        if (v->l->push == line()) {
          v->l->push = v->push;
        } else {
          v->l->push = {
              (int)((v->push.k * 1ll * v->l->push.k) % mod),
              (int)((v->push.k * 1ll * v->l->push.b + v->push.b) % mod)};
        }
        creatend(v->r);
        if (v->r->push == line()) {
          v->r->push = v->push;
        } else {
          v->r->push = {
              (int)((v->push.k * 1ll * v->r->push.k) % mod),
              (int)((v->push.k * 1ll * v->r->push.b + v->push.b) % mod)};
        }
      }
      v->sum = (v->sum * 1ll * v->push.k) % mod;
      v->sum = (v->sum + (r - l) * 1ll * v->push.b) % mod;
      v->sum = (v->sum + mod) % mod;
      v->push = line();
    }
    void update(Node *&v, int l, int r, int li, int ri, line f) {
      creatend(v);
      push(v, l, r);
      if (li >= r || l >= ri) {
        return;
      }
      if (li <= l && r <= ri) {
        v->push = f;
        push(v, l, r);
        return;
      }
      int m = (l + r) / 2;
      update(v->l, l, m, li, ri, f);
      update(v->r, m, r, li, ri, f);
      v->sum = (v->l->sum + v->r->sum) % mod;
    }
    void update(int li, int ri, line f) { update(root, 1, n, li, ri, f); }
    int get() {
      creatend(root);
      push(root, 1, n);
      return root->sum;
    }
  };
  signed main() {
    cin.tie(0)->sync_with_stdio(0);
    int n;
    cin >> n;
    vector<int> a(n);
    cin >> a;
    segtree t;
    t.update(1, a[0] + 1, {0, 1});
    for (int i = 1; i < n; ++i) {
      int sum = t.get();
      t.update(1, a[i] + 1, {-1, sum});
      t.update(a[i] + 1, t.n, {0, 0});
    }
    cout << t.get();
  }
  