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)

# -*- coding:utf-8 -*-

def solve():
    N = int(input())
    Cs = input()

    ans = 0
    left, right = 0, N-1
    while left <= right:
        if Cs[left] == "W" and Cs[right] == "R":
            ans += 1  # 石を入れ替える
            left, right = left+1, right-1
            continue
        elif Cs[left]=="W" and Cs[right]!="R":
            right -= 1
            if left == right: ans += 1  # 石の色を変える
            continue
        elif Cs[left]!="W" and Cs[right]=="R":
            left += 1
            continue
        else:
            left, right = left+1, right-1
            continue

    print(ans)


if __name__ == "__main__":
    solve()