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)

N = int(input())
O = input()

def check(O):
    idx = [i for i, s in enumerate(O) if s == 'R' and i != 0]
    for i in idx:
        if O[i-1] == 'W':
            return False
    return True

def swap(O, t1, t2):
    L = [x for x in O]
    tmp = L[t1]
    L[t1] = L[t2]
    L[t2] = tmp
    return ''.join(L)

count = 0
while not check(O):
    ridx = [i for i, s in enumerate(O) if s == 'R']
    wrdx = [i for i, s in enumerate(O) if s == 'W']
    O = swap(O, max(ridx), min(wrdx))
    count += 1
    
print(count)