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: Accepted)

(async () => {
    let input = '';
    for await (const chunk of process.stdin) input += chunk;
    const values = input.split(/\s/);
    const n = +values.shift();
    let c = values[0].split('').slice(0, n);

    let count = 0;
    let lastRed = n;
    let firstWhite = 0;
    while(true){
        lastRed = c.lastIndexOf('R', lastRed);
        firstWhite = c.indexOf('W', firstWhite);
        if(lastRed < firstWhite)break;
        if(lastRed == -1)break;
        if(firstWhite == -1)break;
        c[lastRed] = 'W';
        c[firstWhite] = 'R';
        count++;
    }
    console.log(count);
})()