Problem:
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

Solution in Ruby (2.7.1) (judge verdict: Wrong Answer)

N = gets.to_i
ary = gets.chomp.split('').reverse
# 全て赤にするケース
cnt_1 = ary.count('W')
# 全て白にするケース
cnt_2 = ary.count('R')
# 最初の白があらわれたところまで全て白、残りは全て赤にするケース
w_idx = ary.index('W')
r_idx = ary[w_idx..-1].index('R')
w_cnt = ary[0..w_idx].count('R') 
r_cnt = w_idx == nil || r_idx == nil ? 0 : ary[(w_idx.to_i+r_idx.to_i)..N-1].count('W')
cnt_3 = [w_cnt, r_cnt].max
puts [cnt_1, cnt_2, cnt_3].min