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())
white=0
red=0
hu=0
W=list(str(input()))
if "W" not in W or "R" not in W:
  print("0")
  exit()
for i in range(N//2):
  if W[i]=="W":
    white+=1
for i in range(N//2):
  if W[N-1-i]=="R":
    red+=1
for k in range(white):
  if W[N-k-1]=="W":
    hu+=1
if hu==white:
  print(0)
  exit()
elif white>=red:
  print(red)
  exit()
else:
  Z=white*2-red
  if (N/2)%1!=0:
    if W[N//2+1]=="W" and W[N//2+2]=="R":
      Z+=1
  print(Z)