Single-use Stones

有许多的青蛙要过河,可惜的是,青蛙根本跳不过河,他们最远只能跳 L 单位长度,而河宽 W 单位长度。 在河面上有一些石头,距离i远的地方有ai个石头,每个石头只能使用一次,求最大能有多少青蛙过河。 输入的第一行为两个整数 W,L (1<l<w<10^5) 第二行有 W-1 个整数a1,a2.......aw-1(0<ai<10^4) 输出为一个整数,即能过河的最大青蛙数

A lot of frogs want to cross a river. A river is $ w $ units width, but frogs can only jump $ l $ units long, where $ l < w $ . Frogs can also jump on lengths shorter than $ l $ . but can't jump longer. Hopefully, there are some stones in the river to help them. The stones are located at integer distances from the banks. There are $ a_i $ stones at the distance of $ i $ units from the bank the frogs are currently at. Each stone can only be used once by one frog, after that it drowns in the water. What is the maximum number of frogs that can cross the river, given that then can only jump on the stones?

The first line contains two integers $ w $ and $ l $ ( $ 1 \le l < w \le 10^5 $ ) — the width of the river and the maximum length of a frog's jump. The second line contains $ w - 1 $ integers $ a_1, a_2, \ldots, a_{w-1} $ ( $ 0 \le a_i \le 10^4 $ ), where $ a_i $ is the number of stones at the distance $ i $ from the bank the frogs are currently at.

Print a single integer — the maximum number of frogs that can cross the river.

10 5
0 0 1 0 2 0 0 1 0

3

10 3
1 1 1 1 2 1 1 1 1

3

In the first sample two frogs can use the different stones at the distance $ 5 $ , and one frog can use the stones at the distances $ 3 $ and then $ 8 $ . In the second sample although there are two stones at the distance $ 5 $ , that does not help. The three paths are: $ 0 \to 3 \to 6 \to 9 \to 10 $ , $ 0 \to 2 \to 5 \to 8 \to 10 $ , $ 0 \to 1 \to 4 \to 7 \to 10 $ .