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: MOD = 998244353
  N, = map(int, input().split())
  X = [0] + list(map(int, input().split()))
  Y = [0]*(N+1)
  
  stack = [(-1, 0)]
  for i in range(1, N+1):
      while stack and stack[-1][0] > X[i]:
          stack.pop()
      Y[i] = stack[-1][1]
      stack.append((X[i], i))
  dp = [0]*(N+1)
  dp[0] = 1
  S = [[0, 0] for _ in range(N+1)]
  for i in range(1, N+1):
      x = X[i]
      k = Y[i]
  
      if i%2:
          dp[i] = (S[i-1][0] - S[k][0])*x
      else:
          dp[i] = (S[i-1][1] - S[k][1])*x
  #    print(i, k, dp[i])
      if k > 0:
          dp[i] += pow(-1, i-k+1)*dp[k]*(x-1)
      else:
          dp[i] += pow(-1, i-k+1)*(dp[k]*x)
      dp[i]%=MOD
      S[i][0] += S[i-1][0] + pow(-1, i)*dp[i]
      S[i][1] += S[i-1][1] + pow(-1, i+1)*dp[i]
      S[i][0] %= MOD
      S[i][1] %= MOD
  #print(S)
  #print(dp)
  print(dp[-1]%MOD)
  