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: Runtime Error)

import sys
## io ##
def IS(): return sys.stdin.readline().rstrip()
def II(): return int(IS())
def MII(): return list(map(int, IS().split()))
from bisect import bisect_left
from collections import deque
#======================================================#
def main():
    N = II()
    cc = list(IS())
    cc_rev = cc[::-1]
    cnt = 0
    indexs = []
    for i, c in enumerate(cc):
        if c == 'W':
            indexs.append(i)
    indexs = deque(indexs)
    if cc.count('W') == 0:
        print(0)
        return 0
    for i, c in enumerate(cc_rev):
        if c == 'R':
            idx = indexs.popleft()
            if i >= idx:
                cc[idx] = 'R'
                cnt += 1
    print(cnt)

if __name__ == '__main__':
    main()