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;
    stack<int> R;
    queue<int> W;
    string s;
    cin>>s;
    int l = s.length();
    for (int i=0; i<l; i++) 
        if (s[i] == 'R') R.push(i);
        else W.push(i);
    int res = 0;
    while(!R.empty() && !W.empty())
    {
        int a = R.top(), b = W.front();
        if (a > b) {
            //cout << a << " " << b << '\n';
            swap(s[a], s[b]); res++;
        }
        else break;
        R.pop(); W.pop();
        
    }
    cout << res << '\n';
    return 0;
}