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 <unordered_map>
#include <unordered_set>
#include <algorithm>
#include <iostream>
#include <iostream>
#include <fstream>
#include <numeric>
#include <cstring>
#include <cassert>
#include <string>
#include <vector>
#include <queue>
#include <deque>
#include <cmath>
#include <set>
#include <map>
#include <functional>
#include <bitset>
#include <iomanip>
#include <stack>
#include <list>
using namespace std;
#define ll           long long
#define all(v)       (v).begin() , (v).end()
#define vi           vector <ll>
#define vii          vector <pair<ll,ll>>
#define ii           pair<ll,ll>
#define sz(v)	     ((int)((v).size()))
#define lp(i,a,b)    for(int i=a;i<b;i++)
#define pb           push_back
#define pf           push_front
#define F            first
#define S            second
#define endl         "\n"
#define IOS          ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
const double PI = 3.14159265358979323846;
const ll inf=2e9,MOD=1e9+7,MAX=1e6+1;

int main() {
#ifndef ONLINE_JUDGE
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
#endif
    int n;
    string s;
    cin>>n>>s;
    set<int>r,w;
    lp(i,0,n){
        if(s[i]=='R')
            r.insert(i);
        else
            w.insert(i);
    }
    int res=0;
    for(auto i:w){
        if(sz(r)){
            int pos=*prev(r.end());
            if(i<pos)
                swap(s[i],s[pos]),r.erase(pos),res++;
        }
    }
    cout<<res;
}