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)

import collections

def solve():
    wr = collections.Counter(c)
    
    w = wr["W"]
    r = wr["R"]
    R = r
    left  = collections.Counter(c[0:r])
    right = collections.Counter(c[r:N])
#    print(left)
#    print(right)
#    print(right["R"])
    cnt = 0
    tmp = 0
    is_change = False
    for i in range(N):
        if not is_change and c[i] == "W":
            cnt += 1
            tmp += 1
            is_change = True
        elif is_change and c[i] == "W":
            cnt += 1
            r += 1
            tmp += 1
            is_change = False
        elif c[i] == "R":
            tmp += 1
        if tmp == r:
            break
#        print(i,cnt,tmp)
    if w==len(c):
        print(1)
    else:
        print(cnt)

if __name__ == "__main__":
    N = int(input())
    C = input()
    c = [str(l) for l in C]
#    print(c)
    solve()