#[allow(unused_imports)]
use std::cmp::*;
#[allow(unused_imports)]
use std::collections::*;
use std::io::*;
#[allow(dead_code)]
fn getline() -> String {
    let mut ret = String::new();
    std::io::stdin().read_line(&mut ret).ok();
    return ret;
}
fn get_word() -> String {
    let mut stdin = std::io::stdin();
    let mut u8b: [u8; 1] = [0];
    loop {
        let mut buf: Vec<u8> = Vec::with_capacity(16);
        loop {
            let res = stdin.read(&mut u8b);
            if res.is_err() || u8b[0] <= ' ' as u8 {
                break;
            } else {
                buf.push(u8b[0]);
            }
        }
        if buf.len() >= 1 {
            let ret = std::string::String::from_utf8(buf).unwrap();
            return ret;
        }
    }
}
fn parse<T: std::str::FromStr>(s: &str) -> T { s.parse::<T>().ok().unwrap() }

#[allow(dead_code)]
fn get<T: std::str::FromStr>() -> T { parse(&get_word()) }

fn try_sort(a: &mut [i32], k: usize, start: usize) -> i32 {
    let n = a.len();
    if start >= n {
        return 0;
    }
    let steps = (n - start - 1) / k + 1;
    let mut cnt = 0;
    for i in (0 .. steps).rev() {
        for j in 0 .. i {
            let ind1 = start + j * k;
            let ind2 = start + (j + 1) * k;
            if a[ind1] > a[ind2] {
                let t = a[ind1];
                a[ind1] = a[ind2];
                a[ind2] = t;
                cnt += 1;
            }
        }
    }
    return cnt;
}
fn main() {
    let n: usize = get();
    let k: usize = get();
    let mut a: Vec<i32> = vec![0; n];
    for i in 0 .. n {
        a[i] = get();
    }
    let mut cnt = 0;
    for i in 0 .. k {
        cnt += try_sort(&mut a, k, i);
    }
    let mut is_sorted = true;
    for i in 0 .. (n - 1) {
        if a[i] > a[i + 1] {
            is_sorted = false;
        }
    }
    println!("{}", if is_sorted { cnt } else { -1 });
}