Problem:
Score : 400 points

Problem Statement

There are N people standing in a queue from west to east.

Given is a string S of length N representing the directions of the people.
The i-th person from the west is facing west if the i-th character of S is L, and east if that character of S is R.

A person is happy if the person in front of him/her is facing the same direction.
If no person is standing in front of a person, however, he/she is not happy.

You can perform the following operation any number of times between 0 and K (inclusive):

Operation: Choose integers l and r such that 1 \leq l \leq r \leq N, and rotate by 180 degrees the part of the queue: the l-th, (l+1)-th, ..., r-th persons. That is, for each i = 0, 1, ..., r-l, the (l + i)-th person from the west will stand the (r - i)-th from the west after the operation, facing east if he/she is facing west now, and vice versa.

What is the maximum possible number of happy people you can have?

Constraints

N is an integer satisfying 1 \leq N \leq 10^5.

K is an integer satisfying 1 \leq K \leq 10^5.

|S| = N

Each character of S is L or R.

Input

Input is given from Standard Input in the following format:

N K
S

Output

Print the maximum possible number of happy people after at most K operations.

Sample Input 1

6 1
LRLRRL

Sample Output 1

3

If we choose (l, r) = (2, 5), we have LLLRLL, where the 2-nd, 3-rd, and 6-th persons from the west are happy.

Sample Input 2

13 3
LRRLRLRRLRLLR

Sample Output 2

9

Sample Input 3

10 1
LLLLLRRRRR

Sample Output 3

9

Sample Input 4

9 2
RRRLRLRLL

Sample Output 4

7

Solution in Rust (1.15.1) (judge verdict: Accepted)

fn read<T: std::str::FromStr>() -> T {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    s.trim().parse().ok().unwrap()
}

fn read_vec<T: std::str::FromStr>() -> Vec<T> {
    read::<String>()
        .split_whitespace()
        .map(|s| s.parse().ok().unwrap())
        .collect()
}

enum Direction {
    Left,
    Right,
}

fn main() {
    use Direction::*;

    let input = read_vec::<usize>();
    let (_n, k) = (input[0], input[1]);
    let chars = read::<String>().chars().collect::<Vec<_>>();

    let mut count = 0;
    let mut rl = 0;
    let mut lr = 0;

    let mut prev: Option<Direction> = None;
    for c in chars {
        match c {
            'R' => {
                match prev {
                    Some(Left) => {
                        lr += 1;
                    },
                    Some(Right) => {
                        count += 1;
                    }
                    None => {}
                }
                prev = Some(Right);
            },
            'L' => {
                match prev {
                    Some(Left) => {
                        count += 1;
                    },
                    Some(Right) => {
                        rl += 1;
                    },
                    None => {}
                }
                prev = Some(Left);
            },
            _ => {},
        }
    }

    for _ in 0..k {
        if lr > 0 && rl > 0 {
            lr -= 1;
            rl -= 1;
            count += 2;
        } else if lr > 0 {
            lr -= 1;
            count += 1;
        } else if rl > 0 {
            rl -= 1;
            count += 1;
        } else {
            break;
        }
    }

    println!("{}", count);
}