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;
  int a[500500], n;
  int main() {
    ios::sync_with_stdio(false);
    cin >> n;
    for (int i = (1); i <= (n); i++) cin >> a[i];
    long long val = 0, sum = 0;
    stack<pair<long long, int> > s;
    for (int i = (1); i <= (n); i++) {
      val = sum + (i == 1);
      (sum += 998244353 - val * a[i] % 998244353) %= 998244353;
      while (!s.empty() && s.top().second > a[i]) {
        pair<long long, int> cur = s.top();
        s.pop();
        (sum += cur.first * (cur.second - a[i])) %= 998244353;
        (val += cur.first) %= 998244353;
      }
      s.push({val, a[i]});
    }
    cout << (n % 2 ? (998244353 - sum) % 998244353 : sum) << endl;
    return 0;
  }
  