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> 
#define rep(a,b) for(int i = a;i < b; i++)
using namespace std;

void troca(int i, int j, char * vet){
	char aux;
	aux = vet[i];
	vet[i] = vet[j];
	vet[j] = aux;
}

int main() {
	int x, res=0,sim1=0, sim2=0, parei, para=0;
	char aux[212345];
	scanf(" %d", &x);
	scanf(" %[^\n]", aux);
	rep(0,x) {
		if(aux[i] == 'W') sim1=1;
		if(aux[i] == 'R') sim2=1;
	}
	if (sim1 and sim2){
		parei = x-1;	
		for (int i = 0;i <= x;i++) {
			// printf("I = %d\n", i);
			para=0;
			if (aux[i] == 'W'){		
				for (int j = parei;j >= i;j--) {
					// printf("J = %d\n", j);
					if (aux[j] == 'R'){
						parei = j-1;
						troca(i,j,aux);
						res++;para=1;
						break;
					}
				}
				if (!para) {
					break;
				}
			}
		}
	}
	// printf("%s\n", aux);
	printf("%d\n", res);
	
	return 0;
}