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())
rw=list(input())
s=len(rw)
i=0
j=s-1
count=0

while i<=j:
  while (rw[i]=='R' and i<s-1):
        i+=1
  while (rw[j]=='W' and j>0):
        j-=1
  if rw[i]=='W' and rw[j]=='R':
      rw[i]='R'
      rw[j]='W'
      count+=1
  i+=1
  j-=1

j=s-1
while (rw[j]=='W' and j>=0):
      j-=1
i=0 
while (i<j):
   if (rw[i]=='W'):
      count+=1
   i+=1  
print(count)