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D - Alter Altar /

Time Limit: 2 sec / Memory Limit: 1024 MB

問題文

あなたは、以下の二種の操作を任意の順に何度でも行うことができます。

• 石を 2 個選び (隣り合っていなくてもよい)、それらを入れ替える。
• 石を 1 個選び、その石の色を変える (赤なら白に、白なら赤に)。

制約

• 2 \leq N \leq 200000
• c_iR または W

入力

N
c_{1}c_{2}...c_{N}


入力例 1

4
WWRR


出力例 1

2


• 左から 1 番目の石と左から 3 番目の石を入れ替え、RWWR とする。
• 左から 4 番目の石の色を変え、RWWW とする。

入力例 2

2
RR


出力例 2

0


入力例 3

8
WRWWRWRR


出力例 3

3


Score : 400 points

Problem Statement

An altar enshrines N stones arranged in a row from left to right. The color of the i-th stone from the left (1 \leq i \leq N) is given to you as a character c_i; R stands for red and W stands for white.

You can do the following two kinds of operations any number of times in any order:

• Choose two stones (not necessarily adjacent) and swap them.
• Choose one stone and change its color (from red to white and vice versa).

According to a fortune-teller, a white stone placed to the immediate left of a red stone will bring a disaster. At least how many operations are needed to reach a situation without such a white stone?

Constraints

• 2 \leq N \leq 200000
• c_i is R or W.

Input

Input is given from Standard Input in the following format:

N
c_{1}c_{2}...c_{N}


Output

Print an integer representing the minimum number of operations needed.

Sample Input 1

4
WWRR


Sample Output 1

2


For example, the two operations below will achieve the objective.

• Swap the 1-st and 3-rd stones from the left, resulting in RWWR.
• Change the color of the 4-th stone from the left, resulting in RWWW.

Sample Input 2

2
RR


Sample Output 2

0


It can be the case that no operation is needed.

Sample Input 3

8
WRWWRWRR


Sample Output 3

3