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: Time Limit Exceeded)

def change(stones):

    whiteind = stones.find('W')
    redind = stones.rfind('R')

    if whiteind == -1:
        return 'end'

    if whiteind > redind:
        return 'end'

    wklist = list(stones)
    wklist[whiteind] = 'R'
    wklist[redind] = 'W'

    return ''.join(wklist)


def main():
    N = int(input())
    stones = input()

    count = 0
    while(count < (N/2)):
        stones = change(stones)
        if stones == 'end':
            break
        count += 1


    print(count)

if __name__ == '__main__':
    main()