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: Accepted)

n = int(input())
C = list(input())
# print(n)
# print(C)
l = 0
r = n-1
c = 0
while l < r:
    # print('l {} r {}'.format(l,r))
    # print('C[l] {} C[r] {}'.format(C[l],C[r]))

    if C[l]=='W' and C[r]=='R':
        C[l],C[r] = C[r],C[l]
        l += 1
        r -= 1
        c += 1
        # print('C {}'.format(C))

    elif C[l]!='W':
        l += 1
    elif C[r]!='R':
        r -= 1

print(c)