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)

from itertools import combinations
import math
N = int(input())
C = input()
#N, D=map(int,input().split())
#C = []
#for i in range(N):
#    C.append(list(map(int,input().split())))
w = -1
r = -1
for i in range(N):
    if C[i] == 'W':
        w = i
        break
for i in reversed(range(N)):
    if C[i] == 'R':
        r = i
        break
if w == -1 or r == -1:
    print(0)
    exit(0)
c = C[w:r+1]
center = c.count('R')-1
wc = c.count('W')
rc = c.count('R')
if wc==rc:
    print(c[0:center+1].count('W'))
elif wc>rc:
    print(c[0:center+1].count('W'))
else:
    print(c[center:-1].count('R'))