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)

import sys

def main():

    input = sys.stdin.readline

    N = int(input())
    c = str(input())[:-1]

    top_index = 0
    bottom_index = N - 1

    ans = 0

    while top_index < bottom_index:
        if(c[top_index] == "W") and (c[bottom_index] == "R"):
            ans += 1
            top_index += 1
            bottom_index -= 1
            continue

        if c[top_index] == "R":
            top_index += 1
        if c[bottom_index] == "W":
            bottom_index -= 1


    print(ans)

if __name__ == "__main__":
    main()