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)

N = int(input())
C = input()

R, W = [], []
R_ = []
nR, nW = 0, 0
for c in C:
    if c == 'R':
        nR += 1
    else:
        nW += 1
    R.append(nR)
    W.append(nW)

for r in R:
    R_.append(nR - r)

#print(R)
#print(W)
#print(R_)

ans = N

for i in range(N):
    r, w, r_ = R[i], W[i], R_[i]
    if r_ > w:
        ans = min(r_, ans)
    else:
        ans = min(w, ans)
print(ans)