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 <iostream>
#include <string>
#include <vector>
using namespace std;
int main(){
    int n;
    cin >> n;
    string s;
    cin >> s;
    int nr = 0;
    vector<int> ad;
    int ans = 0;
    for(int i = 0; i < n; i++){
        if(s[i] == 'R'){
            nr++;
            ad.push_back(i);
        }
    }
    for(int i = 0; i < nr; i++){
        if(s[i] == 'W'){
            s[i] = 'R';
            int a = ad.back();
            ad.pop_back();
            s[a] = 'W';
            ans++;
        }
    }
    cout << ans << endl;
    return 0;
}