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 mod = 998244353;
  long long a[200010], f[200010][2], s[200010][2];
  long long stk[200010], cnt = 0, res[200010][2];
  inline long long p1(long long x) { return (x & 1) ? mod - 1 : 1; }
  signed main() {
    long long n;
    scanf("%lld", &n);
    for (long long i = 1; i <= n; i++) scanf("%lld", &a[i]);
    f[0][0] = s[0][0] = 1, f[0][1] = s[0][1] = 0;
    stk[0] = res[0][0] = res[0][1] = 0;
    for (long long i = 1; i <= n; i++) {
      while (cnt && a[stk[cnt]] >= a[i]) cnt--;
      stk[++cnt] = i;
      f[i][0] =
          ((cnt == 1 ? 0 : f[stk[cnt - 1]][0]) +
           (s[i - 1][1] - (cnt == 1 ? 0 : s[stk[cnt - 1] - 1][1]) + mod) * a[i]) %
          mod;
      f[i][1] =
          ((cnt == 1 ? 0 : f[stk[cnt - 1]][1]) +
           (s[i - 1][0] - (cnt == 1 ? 0 : s[stk[cnt - 1] - 1][0]) + mod) * a[i]) %
          mod;
      s[i][0] = (s[i - 1][0] + f[i][0]) % mod;
      s[i][1] = (s[i - 1][1] + f[i][1]) % mod;
    }
    printf("%lld\n", (f[n][0] - f[n][1] + mod) * p1(n) % mod);
    return 0;
  }
  