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: Accepted)

N = int(input())
stone = list(map(str,input()))
ind = [""]*2
ind[0] = list()
ind[1] = list()
R = 0
W = 0
for i in range(N):
    if(stone[i]=="W"):
        ind[0].append(i)
        W += 1
    else:
        ind[1].append(i)
        R += 1
newR = 0
for i in range(R):
    if(i != ind[1][i]):
        break
    newR+= 1


newind =list(reversed(ind[1]))
ans2 = 0

# print(W,R,newR)
# print(ind[1],newind)
if(newR != R):
    ans1 = min(W,R-newR)
else:
    ans1 = min(W,R)
if(ans1 != 0):
    for i in range(N//2):
        if(ind[0][i]>newind[i]):
            break
        ans2+=1
        # print(ans)
print(min(ans1,ans2))