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: Wrong Answer)

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

const int INF = 1e9;

int main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	cout.tie(0);

    int n;
    cin >> n;
    string s;
    cin >> s;
    vector<int> white(n), red(n);
    for (int i = 0; i < n; i++) {
        if (i) white[i] = white[i - 1];
        if (s[i] == 'R')
            red[i]++;
        else
            white[i]++;
    }
    int resp = white[n - 1];
    for (int i = 0; i < n; i++) {
        int sub = i ? white[i - 1] : 0;
        resp = min(resp, max(i + 1 - red[i], n - white[n - 1] - sub));
    }
    cout << resp << endl;

	return 0;
}