1139B · Chocolates

Problem: You went to the store, selling $n$ types of chocolates. There are $a_i$ chocolates of type $i$ in stock.

You have unlimited amount of cash (so you are not restricted by any prices) and want to buy as many chocolates as possible. However if you buy $x_i$ chocolates of type $i$ (clearly, $0 \le x_i \le a_i$), then for all $1 \le j < i$ at least one of the following must hold:

• $x_j = 0$ (you bought zero chocolates of type $j$)
• $x_j < x_i$ (you bought less chocolates of type $j$ than of type $i$)

For example, the array $x = [0, 0, 1, 2, 10]$ satisfies the requirement above (assuming that all $a_i \ge x_i$), while arrays $x = [0, 1, 0]$, $x = [5, 5]$ and $x = [3, 2]$ don't.

Calculate the maximum number of chocolates you can buy.

Input Format: The first line contains an integer $n$ ($1 \le n \le 2 \cdot 10^5$), denoting the number of types of chocolate.

The next line contains $n$ integers $a_i$ ($1 \le a_i \le 10^9$), denoting the number of chocolates of each type.

Output Format: Print the maximum number of chocolates you can buy.

Note: In the first example, it is optimal to buy: $0 + 0 + 1 + 3 + 6$ chocolates.

In the second example, it is optimal to buy: $1 + 2 + 3 + 4 + 10$ chocolates.

In the third example, it is optimal to buy: $0 + 0 + 0 + 1$ chocolates.