// -*- coding:utf-8-unix -*- // #![feature(map_first_last)] #![allow(dead_code)] #![allow(unused_imports)] #![allow(unused_macros)] use std::arch::x86_64::_MM_FROUND_NEARBYINT; // use core::num; use std::cmp::*; use std::fmt::*; use std::hash::*; use std::iter::FromIterator; use std::*; use std::{cmp, collections, fmt, io, iter, ops, str}; const INF: i64 = 1223372036854775807; const UINF: usize = INF as usize; const LINF: i64 = 2147483647; const INF128: i128 = 1223372036854775807000000000000; const MOD1: i64 = 1000000007; const MOD9: i64 = 998244353; const MOD: i64 = MOD9; // const MOD: i64 = MOD2; const UMOD: usize = MOD as usize; const M_PI: f64 = 3.14159265358979323846; // use proconio::input; // const MOD: i64 = INF; use cmp::Ordering::*; use std::collections::*; use std::io::stdin; use std::io::stdout; use std::io::Write; macro_rules! p { ($x:expr) => { //if expr println!("{}", $x); }; } macro_rules! vp { // vector print separate with space ($x:expr) => { println!( "{}", $x.iter() .map(|x| x.to_string()) .collect::>() .join(" ") ); }; } macro_rules! d { ($x:expr) => { println!("{:?}", $x); }; } macro_rules! yn { ($val:expr) => { if $val { println!("Yes"); } else { println!("No"); } }; } fn main() { solve(); } // use str::Chars; #[allow(dead_code)] fn read() -> T { let mut s = String::new(); std::io::stdin().read_line(&mut s).ok(); s.trim().parse().ok().unwrap() } #[allow(dead_code)] fn readi() -> i64 { let mut str = String::new(); let _ = stdin().read_line(&mut str).unwrap(); let mut iter = str.split_whitespace(); iter.next().unwrap().parse::().unwrap() } #[allow(dead_code)] fn read_vec() -> Vec { read::() .split_whitespace() .map(|e| e.parse().ok().unwrap()) .collect() } #[allow(dead_code)] fn read_mat(n: u32) -> Vec> { (0..n).map(|_| read_vec()).collect() } #[allow(dead_code)] fn readii() -> (i64, i64) { let mut str = String::new(); let _ = stdin().read_line(&mut str).unwrap(); let mut iter = str.split_whitespace(); ( iter.next().unwrap().parse::().unwrap(), iter.next().unwrap().parse::().unwrap(), ) } #[allow(dead_code)] fn readiii() -> (i64, i64, i64) { let mut str = String::new(); let _ = stdin().read_line(&mut str).unwrap(); let mut iter = str.split_whitespace(); ( iter.next().unwrap().parse::().unwrap(), iter.next().unwrap().parse::().unwrap(), iter.next().unwrap().parse::().unwrap(), ) } #[allow(dead_code)] fn readuu() -> (usize, usize) { let mut str = String::new(); let _ = stdin().read_line(&mut str).unwrap(); let mut iter = str.split_whitespace(); ( iter.next().unwrap().parse::().unwrap(), iter.next().unwrap().parse::().unwrap(), ) } #[allow(dead_code)] fn readff() -> (f64, f64) { let mut str = String::new(); let _ = stdin().read_line(&mut str).unwrap(); let mut iter = str.split_whitespace(); ( iter.next().unwrap().parse::().unwrap(), iter.next().unwrap().parse::().unwrap(), ) } fn readcc() -> (char, char) { let mut str = String::new(); let _ = stdin().read_line(&mut str).unwrap(); let mut iter = str.split_whitespace(); ( iter.next().unwrap().parse::().unwrap(), iter.next().unwrap().parse::().unwrap(), ) } fn readuuu() -> (usize, usize, usize) { let mut str = String::new(); let _ = stdin().read_line(&mut str).unwrap(); let mut iter = str.split_whitespace(); ( iter.next().unwrap().parse::().unwrap(), iter.next().unwrap().parse::().unwrap(), iter.next().unwrap().parse::().unwrap(), ) } #[allow(dead_code)] fn readiiii() -> (i64, i64, i64, i64) { let mut str = String::new(); let _ = stdin().read_line(&mut str).unwrap(); let mut iter = str.split_whitespace(); ( iter.next().unwrap().parse::().unwrap(), iter.next().unwrap().parse::().unwrap(), iter.next().unwrap().parse::().unwrap(), iter.next().unwrap().parse::().unwrap(), ) } #[allow(dead_code)] fn readuuuu() -> (usize, usize, usize, usize) { let mut str = String::new(); let _ = stdin().read_line(&mut str).unwrap(); let mut iter = str.split_whitespace(); ( iter.next().unwrap().parse::().unwrap(), iter.next().unwrap().parse::().unwrap(), iter.next().unwrap().parse::().unwrap(), iter.next().unwrap().parse::().unwrap(), ) } #[allow(dead_code)] pub trait Monoid { type T: Clone; fn id() -> Self::T; fn op(a: &Self::T, b: &Self::T) -> Self::T; } #[allow(dead_code)] pub enum SUM {} impl Monoid for SUM { type T = usize; fn id() -> Self::T { 0 } fn op(a: &Self::T, b: &Self::T) -> Self::T { *a + *b } } #[allow(dead_code)] /// Generic Binary Indexed Tree pub struct BIT { buf: Vec, } impl BIT { #[allow(dead_code)] pub fn new(n: usize) -> Self { Self { buf: vec![M::id(); n + 1], } } #[allow(dead_code)] pub fn sum(&self, i: usize) -> M::T { let mut i = i; let mut s = M::id(); while i > 0 { s = M::op(&s, &self.buf[i]); i &= i - 1; } s } #[allow(dead_code)] pub fn add(&mut self, i: usize, x: &M::T) { let mut i = i as i64; while i < self.buf.len() as i64 { let t = &mut self.buf[i as usize]; *t = M::op(&t, x); i += i & -i; } } } // 0 ~ n -1 fn inversion_number(v: &Vec) -> usize { let mut res = 0; let n = v.len(); let mut bit: BIT = BIT::new(n); let mut res = 0 as usize; for i in 0..n { res += bit.sum((n - v[i])); bit.add(n - v[i], &1); } res } fn solve() { let (n, m) = readuu(); let mut vec: Vec = read_vec(); vec.iter_mut().for_each(|x| *x -= 1); let inv_num = inversion_number(&vec); if n == 2 { if inv_num % 2 != m % 2 { p!("-1"); return; } } if m == 1 { p!(inv_num); return; } let mut ans = (inv_num + m - 1) / m * m; if ans % 2 == 1 && m % 2 == 1 { ans += m; } else if ans % 2 == 1 { p!(-1); return; } p!(ans); return; }