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++ (GCC 9.2.1) (judge verdict: Accepted)

#include <bits/stdc++.h>
using namespace std;

// 
int main() {
    int N;
    cin >> N;
    bool c[N];
    string tmp;
    cin >> tmp;
    for (int i = 0; i < N; i++) {
        if (tmp[i] == 'R') {
            c[i] = false;  // Red
        } else {
            c[i] = true;  // White
        }
    }

    int changeCount = 0;
    int left = 0, right = N - 1;
    while (left < right) {
        // cout << left << " " << right << endl;
        // cout << "true:" << c[left] << " " << c[right] << endl;
        if (c[left] == true && c[right] == false) {
            // cout << "change" << endl; // debug
            swap(c[left], c[right]);
            changeCount++;
        } else if (c[left] == true) {
            right--;
        } else {
            left++;
        }
    }
    cout << changeCount;
}