# Chocolates

## 题目描述

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.

## 输入输出格式

### 输入格式

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.

### 输出格式

Print the maximum number of chocolates you can buy.

5
1 2 1 3 6

10

5
3 2 5 4 10

20

4
1 1 1 1

1

## 说明

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.