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 Python (3.8.2) (judge verdict: Accepted)

# うまく行けば２こ一気に解消できるケースがある
# RRRRWWWWをつくれば良い，一番右のRと一番左のWを入れ替えていく
N = int(input())
C = input()
RL=[]
WL=[]
for i,c in enumerate(C):
    if c == 'R':
        RL.append(-i)
    else:
        WL.append(i)

import heapq
heapq.heapify(RL)
heapq.heapify(WL)
count = 0
if not RL or not WL:
    print(0)
    exit()

while True:
    r = heapq.heappop(RL)
    r = -r
    w = heapq.heappop(WL)
    if w>r:
        print(count)
        exit()
    heapq.heappush(RL,-w)
    heapq.heappush(WL,r)
    count+=1