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 long long maxn = 2e5 + 10;
  const long long mod = 998244353;
  long long dp[maxn], sum[maxn], s[maxn], st[maxn], a[maxn], n;
  signed main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    cin >> n;
    long long ans = 0, top = 0;
    dp[0] = sum[0] = 1;
    for (long long i = 1; i <= n; i++) {
      cin >> a[i];
      while (top && a[st[top]] > a[i]) ans -= s[top--];
      long long gg = sum[i - 1] * a[i];
      if (st[top]) gg -= (sum[st[top] - 1] * a[i]);
      gg %= mod;
      (ans += gg) %= mod;
      st[++top] = i;
      s[top] = gg;
      dp[i] = ans;
      if (!(i & 1)) dp[i] *= -1ll;
      sum[i] = sum[i - 1];
      if (i & 1)
        (sum[i] -= dp[i]) %= mod;
      else
        (sum[i] += dp[i]) %= mod;
    }
    cout << (dp[n] + mod * mod) % mod;
  }
  