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: Wrong Answer)

N = int(input())
c = list(input())

Nw, Nr = 0, 0
for i in range(len(c)):
    Nw += 1 if c[i] == "W" else 0
    Nr += 1 if c[i] == "R" else 0

left_w = 0
right_r = 0
if c[0] == "W":
    left_w = 1
    right_r = Nr
else:
    left_w = 0
    right_r = Nr - 1

#仕切りの位置をiの横とすると
ans = max(left_w, right_r)
for i in range(len(c)):
    if i > 0:
        if c[i] == "W":
            left_w += 1
        else:
            right_r -= 1
    swap = max(left_w, right_r)
    ans = min(swap, ans)
print(ans)