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;
using ll=long long;
#define rep(i,n) for(int i=0;i<(int)(n);i++)
const int INF =1001001001;
using P = pair<int,int>;

int main(void) 
{
    int n;
    cin >> n;
    string c;
    cin >> c;

    ll ans = 0;
    int index = 0;
    rep(i,n){
        if(c[n-i-1] == 'R') 
        {
            for(int j = index; j < n-i-1; j++)
            {
                if(c[j] == 'W')
                {
                    c[j] = 'R';
                    c[n-i-1] = 'W';
                    ans++;
                    break;
                }
                index++;
            }
            if(c[n-i-1] == 'R') break;
            continue;
        }
    }

   cout << ans << endl;
}