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: Runtime Error)

n = int(input())
s = list(input())

l_tmp = 1
r_tmp = -1
# カウント
count = 0
# デバック
# print("    ", s)
# RRRRWW みたいにRを左におしやりたい
for i in range(n):
    if l_tmp - r_tmp - 1 >= n:
        break

    if s[l_tmp - 1] == "R":
        l_tmp += 1
        # print("cont", s)
        continue

    if s[l_tmp - 1] == "W":
        if s[r_tmp] == "R":
            # 交換処理
            s[l_tmp - 1] = "R"
            s[r_tmp] = "W"
            #
            l_tmp += 1
            r_tmp -= 1
            # カウント
            count += 1
            # デバック
            # print("chan", s)
            continue

        if s[r_tmp] == "W":
            # print("-------")
            # print(l_tmp, r_tmp)
            # print(s[l_tmp], s[r_tmp])
            # print("-------")
            # TODO: ここが更新処理で決めつけられない
            while s[r_tmp] == "W":
                r_tmp -= 1
                # print(s)

            # もしなければ更新処理
            if l_tmp - r_tmp - 1 == n and s[r_tmp] == "W":
                # 更新処理
                s[l_tmp - 1] = "R"
                #
                l_tmp += 1
                # カウント
                count += 1
                # デバック
                # print("chan", s)
                continue
            # あれば交換処理

print(count)