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 main(N, S):
    ans = 0
    cw = S.count("W")
    cr = S.count("R")
    cht = min(cw, cr)
    right = N-1
    for left in range(N):
        if S[left] == "W" and S[right] == "R":
            S[left], S[right] = S[right], S[left]
            ans += 1
        while left < right and S[left] == "W" and S[right] != "R":
            right -= 1

        if left >= right or S[left] == "W":
            break
    add = 0
    T = S[:]
    for i in range(N-1):
        if S[i] == "W" and S[i+1] == "R":
            S[i+1] = "W"
            add += 1
    add2 = 0
    for i in range(N-1)[::-1]:
        if T[i] == "W" and T[i+1] == "R":
            T[i] = "R"
            add2 += 1
    print(ans + min(add, add2))


if __name__ == '__main__':
    N = int(input())
    S = [s for s in input()]
    main(N, S)