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
#define ull unsigned long long
#define f first
#define s second
#define vi vector<int>
#define vll vector<ll>
#define pii pair<int,int>
#define vpii vector<pair<int,int>> 
#define pb push_back

void solve() {
	int n;
	cin>>n;
	string s;
	cin>>s;
	int i=0,j=n-1;
	int cnt=0;
	int r1=0,w1=0;
	for(int i=0;i<n;i++) {
		if(s[i]=='R') r1++;
		else w1++;
	}
	while(i<j&&i<n&&j>=0) {
		if(s[i]=='W') {
			while(j>=0&&s[j]!='R') {
				j--;
			}
			if(j>=0&&j>i) {
				swap(s[j],s[i]);
				cnt++;
			}
		}
		i++;
	}
	//cout<<s<<"\n";
	j=n-1;
	while(j>=0&&s[j]!='R') j--;
	for(i=0;i<=j;i++) {
		if(s[i]=='W') {
			cnt++;
			s[i]='-';
		}
	}
	//cout<<s<<"\n";
	cout<<cnt<<"\n";
}
 
int main() {
    #ifndef ONLINE_JUDGE
    freopen("input.txt","r",stdin);
    //freopen("ou.txt","w",stdout);
    #endif
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    int tc=1;
    //cin>>tc;
    for(int t=1;t<=tc;t++) {
        //cout<<"Case #"<<t<<": ";
        solve();
    }
    return 0;
}