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: Wrong Answer)

import sys
from collections import deque

sys.setrecursionlimit(10 ** 7)
f_inf = float('inf')
mod = 10 ** 9 + 7


def resolve():
    n = int(input())
    C = list(input())

    W = deque()
    R = deque()
    flg = False
    for i in range(n):
        if C[i] == "W":
            W.append(i)
            flg = True
        if flg and C[i] == "R":
            R.appendleft(i)

    res = 0
    i = 0
    cnt = 0
    while R:
        if C[i] == "W":
            cnt += 1
        if cnt:
            r = R.popleft()
            w = W.popleft()
            C[r], C[w] = C[w], C[r]
            cnt -= 1
            res += 1
        else:
            R.pop()
        i += 1
    print(res)


if __name__ == '__main__':
    resolve()