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: Time Limit Exceeded)

package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
)

const debug = false

var sc = bufio.NewScanner(os.Stdin)

/*-----------Main-----------*/

func main() {
	N := readI()
	c := readS()

	ans := make([]int, N+1)
	for i := 0; i <= N; i++ {
		rtow, wtor := 0, 0
		for j := 0; j < N; j++ {
			if j >= i {
				if c[j] == 'R' {
					rtow++
				}
			} else {
				if c[j] == 'W' {
					wtor++
				}
			}
		}
		if rtow >= wtor {
			ans[i] = rtow
		} else {
			ans[i] = wtor
		}
	}
	fmt.Println(min(ans))
}

func min(a []int) int {
	m := a[0]
	len := len(a)
	for i := 1; i < len; i++ {
		if m > a[i] {
			m = a[i]
		}
	}
	return m
}

/*-----------Utilities-----------*/

func init() {
	if debug {
		testFile, err := os.Open("./test/sample-3.in")
		if err == nil {
			sc = bufio.NewScanner(testFile)
		}

	}
	const maxBuf = 200100
	var buf []byte = make([]byte, maxBuf)
	sc.Split(bufio.ScanWords)
	sc.Buffer(buf, maxBuf)
}

func readS() string {
	sc.Scan()
	return sc.Text()
}

func readI() int {
	sc.Scan()
	r, _ := strconv.Atoi(sc.Text())
	return r
}

func readF() float64 {
	sc.Scan()
	r, _ := strconv.ParseFloat(sc.Text(), 64)
	return r
}