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: Runtime Error)

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

int main(){
    int N;
    string S;
    cin >> N;
    cin >> S;

    priority_queue<int> red;
    priority_queue<int, vector<int>, greater<int>> white;
    for(int i=0; i<N; i++){
        if(S[i]=='R'){
            red.push(i);
        }else{
            white.push(i);
        }
    }

    int min_index_white = N;
    int max_index_red = -1;
    int count = 0;
    while(!white.empty()){
        min_index_white = white.top();
        max_index_red = red.top();

        if(min_index_white > max_index_red){
            break;
        }

        white.pop();
        red.pop();
        count++;
    }

    cout << count << endl;
    return 0;
}