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 Go (1.14.1) (judge verdict: Wrong Answer)

package main

import (
	"fmt"
	"math"
)

func main() {
	// Code for D - Alter Altar
	var N int
	fmt.Scanf("%d", &N)

	var S string
	fmt.Scanf("%s", &S)

	var flag1 int = 0
	var flag2 int = 0
	var count1, count2 int
	for i := 0; i < N; i++ {
		if flag1 == 0 && S[i] == 'W' {
			count1++
		} else if flag1 == 0 && S[i] == 'R' {
			flag1 = 1
		}

		if flag1 == 1 && S[i] == 'W' {
			flag2 = 1
		}

		if flag1 == 1 && flag2 == 1 && S[i] == 'R' {
			count2++
		}
	}
	if count2 >= count1 {
		fmt.Println(count2)
	} else {
		fmt.Println(int(math.Abs(float64(count1 - count2))))
	}
}