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)

n = int(input())
c = input()
cnt_w = c.count("W")
cnt_r = 0
for i in range(n-1, -1, -1):
    if c[i] == "R":
        cnt_r += 1
    else:
        break
c = list(c)
res = -1
ans = 0
for i in range(n):
    if c[i] == "W":
        cnt_w -= 1
        if cnt_r:
            cnt_r -= 1
            c[res] = "W"
            res -= 1
            ans +=1
            cnt_w += 1
        else:
            ans += 1
    if cnt_w == n-i-1:
        break
print(ans)