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 C++ (Clang 10.0.0) (judge verdict: Time Limit Exceeded)

#include <iostream>
#include <string>

using namespace std;

int main(void)
{
    int N;
    cin >> N;

    char c[N + 1];
    c[0] = 'R';
    for (int i = 1; i <= N; i++)
    {
        cin >> c[i];
    }

    int counter = 0;
    for (int i = N; i != 0; i--)
    {
        if (c[i] == 'R')
        {
            for (int j = 0; j < i; j++)
            {
                if (c[j] == 'W')
                {
                    c[j] = 'R';
                    c[i] = 'W';
                    counter++;
                    break;
                }
            }
        }
    }
    cout << counter << "\n";
}