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)

#!/usr/bin/env python3
n = int(input())
t = input()
*c, = map(int, t.replace(*"W1").replace(*"R0"))

"""
1100 -> 0101 -> 0011
00
10110100 -> 00111100 -> 00011110 -> 00001111
"""

a = 0
i, j = 0, 1
while i + j < n:
    if c[i] == 1 and c[-j] == 0:
        a += 1
        i += 1
        j += 1
    elif c[i] == c[-j] == 0:
        i += 1
    elif c[i] == c[-j] == 1:
        j += 1
print(a)