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;

#define ll long long 

int main() {
    ios::sync_with_stdio(0); 

    int n; cin >> n;
    string s; cin >> s;

    int left = 0;
    int right = n-1;
    int cnt = 0;
    while (left <= right) {
        if (s[left] == 'W') {
            while (right > left and s[right] == 'W') right--;
            if (right == left) break;
            swap(s[left], s[right]);
            cnt++;
        }
        left++;
    }

    //cout << "s: " << s << endl;
    //cout << "left: " << left << endl;
    for (int i = left; i < n; i++) {
        cnt += (s[i] == 'R');
    }

    cout << cnt << endl;



    return 0; 
}