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: import sys
  
  # sys.setrecursionlimit(200005)
  int1 = lambda x: int(x)-1
  pDB = lambda *x: print(*x, end="\n", file=sys.stderr)
  p2D = lambda x: print(*x, sep="\n", end="\n\n", file=sys.stderr)
  def II(): return int(sys.stdin.readline())
  def LI(): return list(map(int, sys.stdin.readline().split()))
  def LLI(rows_number): return [LI() for _ in range(rows_number)]
  def LI1(): return list(map(int1, sys.stdin.readline().split()))
  def LLI1(rows_number): return [LI1() for _ in range(rows_number)]
  def SI(): return sys.stdin.readline().rstrip()
  dij = [(0, 1), (-1, 0), (0, -1), (1, 0)]
  # dij = [(0, 1), (-1, 0), (0, -1), (1, 0), (1, 1), (1, -1), (-1, 1), (-1, -1)]
  inf = 18446744073709551615
  # inf = 4294967295
  # md = 10**9+7
  md = 998244353
  
  n = II()
  aa = LI()
  
  dp = [0]*(n+1)
  dp[0] = 1
  stack = []
  s = 0
  for i, a in enumerate(aa):
      t = -dp[i]
      while stack and stack[-1][0] >= a:
          pa, v = stack.pop()
          s -= pa*v
          t += v
          s %= md
          t %= md
      s += t*a%md
      s %= md
      dp[i+1] = s
      stack.append((a, t))
  
  ans = dp[-1]
  if n & 1: ans = -ans%md
  
  print(ans)
  