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: 
  """
  
  from
  https://atcoder.jp/contests/arc115/submissions/27470789
  
  """
  
  import sys
  from sys import stdin
  
  input = sys.stdin.readline
  N = int(input())
  a = list(map(int, input().split()))
  mod = 998244353
  
  dp = [0] * (N + 1)
  dp[0] = 1
  s = []
  sm = 0
  for i in range(N):
      x = a[i]
  
      c = dp[i]
      if i & 1: c = -dp[i]
  
      while len(s) and s[-1][0] >= x:
          v, cc = s.pop()
          c += cc
          c %= mod
          sm -= v * cc
          sm %= mod
  
      s.append((x, c))
      sm += x * c
      sm %= mod
  
      if i & 1: dp[i + 1] -= sm
      else: dp[i + 1] += sm
      dp[i + 1] %= mod
  
  print(dp[-1])
  