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 PyPy3 (7.3.0) (judge verdict: Wrong Answer)

def R_W(val: str, n: int) -> int:
    r_lst = []
    for i in range(n):
        if val[i] == 'R':
            r_lst.append(i)
    cou_r = len(r_lst)
    ans = 0
    if cou_r == 0:
        return ans
    dld = 0
    for i in range(cou_r):
        cou_w = r_lst[i] - dld
        if cou_w >= cou_r - 1:
            ans += cou_w
            return ans
        else:
            cou_r -= 1 + r_lst[i]
            dld += cou_w + 1
            ans += cou_w

n = int(input())
c = input()

print(R_W(c, n))