A14623. Non-equal Neighbours

You are given an array of $n$ positive integers $a_1, a_2, \ldots, a_n$ . Your task is to calculate the number of arrays of $n$ positive integers $b_1, b_2, \ldots, b_n$ such that:

- $1 \le b_i \le a_i$ for every $i$ ( $1 \le i \le n$ ), and
- $b_i \neq b_{i+1}$ for every $i$ ( $1 \le i \le n - 1$ ).

The number of such arrays can be very large, so print it modulo $998\,244\,353$ .

The first line contains a single integer $n$ ( $1 \le n \le 2 \cdot 10^5$ ) — the length of the array $a$ .

The second line contains $n$ integers $a_1, a_2, \ldots, a_n$ ( $1 \le a_i \le 10^9$ ).

Print the answer modulo $998\,244\,353$ in a single line.

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 $[2, 3]$ .