Anatoly lives in the university dorm as many other students do. As you know, cockroaches are also living there together with students. Cockroaches might be of two colors: black and red. There are _n_ cockroaches living in Anatoly's room.
Anatoly just made all his cockroaches to form a single line. As he is a perfectionist, he would like the colors of cockroaches in the line to **alternate**. He has a can of black paint and a can of red paint. In one turn he can either swap any two cockroaches, or take any single cockroach and change it's color.
Help Anatoly find out the minimum number of turns he needs to make the colors of cockroaches in the line alternate.
## Input
The first line of the input contains a single integer _n_ (1 ≤ _n_ ≤ 100 000) — the number of cockroaches.
The second line contains a string of length _n_, consisting of characters '_b_' and '_r_' that denote black cockroach and red cockroach respectively.
## Output
Print one integer — the minimum number of moves Anatoly has to perform in order to make the colors of cockroaches in the line to alternate.
[samples]
## Note
In the first sample, Anatoly has to swap third and fourth cockroaches. He needs 1 turn to do this.
In the second sample, the optimum answer is to paint the second and the fourth cockroaches red. This requires 2 turns.
In the third sample, the colors of cockroaches in the line are alternating already, thus the answer is 0.
Anatoly 住在大学宿舍里，就像许多其他学生一样。你知道，蟑螂也和学生一起生活在那里。蟑螂有两种颜色：黑色和红色。Anatoly 的房间里有 #cf_span[n] 只蟑螂。
Anatoly 让所有蟑螂排成一条直线。由于他追求完美，他希望这条线上的蟑螂颜色能够*交错*排列。他有一罐黑色油漆和一罐红色油漆。在一次操作中，他可以交换任意两只蟑螂的位置，或者取任意一只蟑螂并改变它的颜色。
请帮助 Anatoly 找出使蟑螂颜色交错排列所需的最少操作次数。
输入的第一行包含一个整数 #cf_span[n] (#cf_span[1 ≤ n ≤ 100 000]) —— 蟑螂的数量。
第二行包含一个长度为 #cf_span[n] 的字符串，由字符 '_b_' 和 '_r_' 组成，分别表示黑色蟑螂和红色蟑螂。
请输出一个整数 —— Anatoly 为使蟑螂颜色交错排列所需执行的最少操作次数。
在第一个样例中，Anatoly 需要交换第三只和第四只蟑螂。他需要 #cf_span[1] 次操作完成。
在第二个样例中，最优解是将第二只和第四只蟑螂涂成红色。这需要 #cf_span[2] 次操作。
在第三个样例中，蟑螂的颜色已经交错排列，因此答案是 #cf_span[0]。
## Input
输入的第一行包含一个整数 #cf_span[n] (#cf_span[1 ≤ n ≤ 100 000]) —— 蟑螂的数量。第二行包含一个长度为 #cf_span[n] 的字符串，由字符 '_b_' 和 '_r_' 组成，分别表示黑色蟑螂和红色蟑螂。
## Output
请输出一个整数 —— Anatoly 为使蟑螂颜色交错排列所需执行的最少操作次数。
[samples]
## Note
在第一个样例中，Anatoly 需要交换第三只和第四只蟑螂。他需要 #cf_span[1] 次操作完成。在第二个样例中，最优解是将第二只和第四只蟑螂涂成红色。这需要 #cf_span[2] 次操作。在第三个样例中，蟑螂的颜色已经交错排列，因此答案是 #cf_span[0]。
**Definitions**
Let $ n \in \mathbb{Z} $, $ 1 \leq n \leq 100{,}000 $, be the number of cockroaches.
Let $ s \in \{b, r\}^n $ be the initial sequence of cockroach colors.
**Constraints**
The target configurations are the two alternating patterns:
- $ P_1 = (b, r, b, r, \dots) $ — starting with black.
- $ P_2 = (r, b, r, b, \dots) $ — starting with red.
**Operations**
In one move, Anatoly may:
1. Swap any two cockroaches, or
2. Change the color of any single cockroach.
**Objective**
Compute the minimum number of moves required to transform $ s $ into either $ P_1 $ or $ P_2 $.
Let $ d_1 $ be the minimum number of moves to convert $ s $ to $ P_1 $, and $ d_2 $ to $ P_2 $.
Then the answer is $ \min(d_1, d_2) $.
For a target pattern $ P \in \{P_1, P_2\} $, define:
- Let $ x $ be the number of positions $ i $ where $ s_i \neq P_i $ and $ s_i = b $, $ P_i = r $.
- Let $ y $ be the number of positions $ i $ where $ s_i \neq P_i $ and $ s_i = r $, $ P_i = b $.
Then $ d = \max(x, y) $, since each swap fixes two mismatches, and any remaining mismatch is fixed by a color change.