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;
  const int dr[]{-1, -1, 0, 1, 1, 1, 0, -1};
  const int dc[]{0, 1, 1, 1, 0, -1, -1, -1};
  void run() {
    ios::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);
  }
  const int MOD = 998244353;
  struct modint {
    int val;
    modint() { val = 0; }
    modint(const long long &a) {
      val = a % MOD;
      if (val < 0) val += MOD;
    }
    modint &operator+=(const modint &a) {
      if ((val += a.val) >= MOD) val -= MOD;
      return (*this);
    }
    modint operator+(const modint &a) const {
      modint c = (*this);
      c += a;
      return c;
    }
    modint &operator-=(const modint &a) {
      if ((val -= a.val) < 0) val += MOD;
      return (*this);
    }
    modint operator-(const modint &a) const {
      modint c = (*this);
      c -= a;
      return c;
    }
    modint operator*(const modint &a) const {
      return modint(1LL * this->val * a.val);
    }
    modint &operator*=(const modint &a) {
      (*this) = (*this) * a;
      return (*this);
    }
    modint operator/(const modint &a) { return (*this) * power(a, MOD - 2); }
    modint &operator/=(const modint &a) {
      (*this) = (*this) / a;
      return (*this);
    }
    static modint power(modint x, long long y) {
      modint res = 1;
      while (y > 0) {
        if (y & 1) res *= x;
        x *= x;
        y >>= 1;
      }
      return res;
    }
    friend ostream &operator<<(ostream &out, const modint &a) {
      out << a.val;
      return out;
    }
  };
  struct segtree {
    segtree *left = nullptr, *right = nullptr;
    modint sum = 0, lazy = 0, sign = 1;
    segtree() {}
    void extend() {
      if (left == nullptr) {
        left = new segtree();
        right = new segtree();
      }
    }
    void pushdown(int start, int end) {
      if (lazy.val == 0 && sign.val == 1) return;
      sum = lazy * (end - start + 1) + sign * sum;
      if (start != end) {
        extend();
        left->sign *= sign;
        left->lazy = lazy + sign * left->lazy;
        right->sign *= sign;
        right->lazy = lazy + sign * right->lazy;
      }
      lazy = 0;
      sign = 1;
    }
    void pushup() { sum = left->sum + right->sum; }
    ~segtree() {
      if (left == nullptr) return;
      delete left;
      delete right;
    }
  };
  class extened_segment_tree {
    void update(segtree *root, int start, int end, int l, int r, int sign,
                int val) {
      root->pushdown(start, end);
      if (r < start || end < l) return;
      if (l <= start && end <= r) {
        root->sign = sign;
        root->lazy = val;
        root->pushdown(start, end);
        return;
      }
      root->extend();
      update(root->left, start, ((start + end) >> 1), l, r, sign, val);
      update(root->right, ((start + end) >> 1) + 1, end, l, r, sign, val);
      root->pushup();
    }
  
   public:
    int start, end;
    segtree *root;
    extened_segment_tree() {}
    extened_segment_tree(int start, int end) : start(start), end(end) {
      root = new segtree();
    }
    void update(int l, int r, int sign, int val) {
      update(root, start, end, l, r, sign, val);
    }
    int query() {
      root->pushdown(start, end);
      return root->sum.val;
    }
  };
  const int N = 1e9;
  int main() {
    run();
    int n;
    cin >> n;
    vector<int> a(n);
    for (auto &it : a) cin >> it;
    extened_segment_tree seg(1, N);
    seg.update(1, a[0], 1, 1);
    for (int i = 1; i < n; i++) {
      int sum = seg.query();
      seg.update(1, a[i], -1, sum);
      seg.update(a[i] + 1, N, 0, 0);
    }
    cout << seg.query() << "\n";
  }
  