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: Accepted)

def main():
    N = int(input())
    c = input()
    ans = c.count('W')
    if ans == 0 or ans == N:
        print(0)
        exit()
    
    Ws = []
    Rs = []
    for i in range(N):
        if c[i] == 'W':
            Ws.append(i)
        if c[N-1-i] == 'R':
            Rs.append(N-1-i)

    Ws.append(N+1)
    Rs.append(-1)
    tmp = 0
    while Ws[tmp] < Rs[tmp]:
        tmp += 1
    
    print(min(ans,tmp))

if __name__ == "__main__":
    main()