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: Runtime Error)

n = int(input())
c = input()
cl = list(c)
wi = 0
wnum=0
ri = n-1
rnum=0
br=0
bw=0
for i in range(n):
    while(cl[wi] == "R" and bw==0):
        wi+=1
        if(wi >= n-1):
            bw=1
            break
    if(cl[wi] == "W"):
        wnum+=1
    wi+=1
    if(wi >= n-1):
        wi=0
        bw=1
    while(cl[ri] == "W" and br==0):
        ri-=1
        if(ri <= 0):
            br=1
            break
    if(cl[ri] == "R"):
        rnum+=1
    ri-=1
    if(ri <= 0):
        ri=n-1
        br=1
    if(ri < wi):
        res1 = i+1
    if(br*bw==1):
        break
print(min(res1, rnum, wnum))