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 JavaScript (Node.js 12.16.1) (judge verdict: Wrong Answer)

function MainD(input){
    const arr = input.split('\n');
    const N = arr[0];
    const Ci = arr[1].split('');
    let answer = 0;

    // Wの数を数える
    let Wnum = 0;
    for(let i=0; i<Ci.length; i++){
        if(Ci[i] === 'W'){
            Wnum++;
        }
    }
    if(Wnum===0) return console.log(0); // Wが現れなかったら0を出力

    // Wの数以降のRの数を数える
    for(let i=Wnum-1; i<Ci.length; i++){
        if(Ci[i] === 'R'){
            answer++;
        }
    }
    console.log(answer);
}
MainD(require("fs").readFileSync("/dev/stdin", "utf8"));