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>
#include <string>
#include <cstdint>
#include <iterator>
#include <algorithm>
#include <fstream>
#include <unordered_map>

using namespace std;

#define FAST_IO ios::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
#define Int int_fast64_t

Int count_fun(string temp,Int n,char c)
{
    Int count=0,i=0;
    if(n==0)
        return 0;
    for(Int i=0;i<n;++i)
    {
        if(temp[i]==c)
            count++;
    }
    return count;
}

Int helper(Int n,string temp)
{
    Int count=0,pos=n-1;
    Int fun=count_fun(temp,n,'R');
    Int fun2=count_fun(temp,fun,'W');
    return fun2;
}

int main()
{
    FAST_IO;

    Int n,count=0;
    string str;
    cin >> n;
    cin >> str;
    count=helper(n,str);
    cout << count << endl;

    return 0;
}