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>
  const int maxn = 200005, mod = 998244353;
  int n, top, ans;
  int a[maxn], stk[maxn][2];
  int main() {
    scanf("%d", &n);
    for (int i = 1; i <= n; i++) {
      scanf("%d", &a[i]);
      int val = (ans + (i == 1)) % mod;
      ans = (ans - 1ll * val * a[i] % mod + mod) % mod;
      while (top > 0 && stk[top][1] > a[i])
        ans = (ans + 1ll * stk[top][0] * (stk[top][1] - a[i])) % mod,
        val = (val + stk[top][0]) % mod, top--;
      stk[++top][0] = val, stk[top][1] = a[i];
    }
    printf("%d\n", n % 2 == 0 ? ans : (mod - ans) % mod);
    return 0;
  }
  