#![allow(non_snake_case)]
#![allow(unused_imports)]
#![allow(unused_macros)]
#![allow(clippy::needless_range_loop)]
#![allow(clippy::comparison_chain)]
#![allow(clippy::nonminimal_bool)]
#![allow(clippy::neg_multiply)]
#![allow(dead_code)]
use std::cmp::{min, Reverse};
use std::collections::{vec_deque, BTreeMap, BTreeSet, BinaryHeap, VecDeque};

const MOD: usize = 998244353;

#[derive(Default)]
struct Solver {}
impl Solver {
    fn solve(&mut self) {
        input! {
            N: usize,
            M: usize,
            P: [usize; N]
        }
        let mut seg = SegmentTree::new(N + 1);
        let mut cnt = 0_usize;

        for &p in &P {
            cnt += seg.query(p, N + 1, 1, seg.offset + 1, 1) as usize;
            seg.update(p - 1, 1);
        }

        if cnt % 2 == 1 && M % 2 == 0 {
            println!("-1");
        } else {
            let target = M * ceil(cnt, M);
            if cnt % 2 == 1 {
                if target % 2 == 1 {
                    println!("{}", target);
                } else {
                    println!("{}", target + M);
                }
            } else {
                if M % 2 == 0 || target % 2 == 0 {
                    println!("{}", target);
                } else {
                    println!("{}", target + M);
                }
            }
        }
    }
}

fn ceil(a: usize, b: usize) -> usize {
    (a + b - 1) / b
}

#[derive(Debug, Clone)]
struct SegmentTree {
    offset: usize,
    data: Vec<isize>,
}

impl SegmentTree {
    fn new(n: usize) -> Self {
        let mut offset = 1;
        while offset < n {
            offset *= 2;
        }
        let data = vec![0; offset * 2];
        SegmentTree { offset, data }
    }
    fn update(&mut self, pos: usize, x: isize) {
        let mut pos = pos + self.offset;
        self.data[pos] = x;
        while pos > 1 {
            pos /= 2;
            self.data[pos] = self.data[pos * 2] + self.data[pos * 2 + 1];
        }
    }
    // u: current cell number
    // (a, b], (l, r]
    fn query(&self, l: usize, r: usize, a: usize, b: usize, u: usize) -> isize {
        if r <= a || b <= l {
            return 0;
        }
        if l <= a && b <= r {
            return self.data[u];
        }
        let m = (a + b) / 2;
        self.query(l, r, a, m, u * 2) + self.query(l, r, m, b, u * 2 + 1)
    }
}

#[derive(Debug, Clone)]
struct Comb {
    fact: Vec<ModInt>,
    fact_inverse: Vec<ModInt>,
}

impl Comb {
    fn new(n: usize) -> Self {
        let mut fact = vec![Mod::one(), Mod::one()];
        let mut fact_inverse = vec![Mod::one(), Mod::one()];
        let mut inverse = vec![Mod::zero(), Mod::one()];
        for i in 2..=n {
            fact.push(*fact.last().unwrap() * Mod::new(i));
            inverse.push((Mod::zero() - inverse[MOD % i]) * Mod::new(MOD / i));
            fact_inverse.push(*fact_inverse.last().unwrap() * *inverse.last().unwrap());
        }
        Comb { fact, fact_inverse }
    }
    fn nCr(&self, n: usize, r: usize) -> ModInt {
        self.fact[n] * self.fact_inverse[n - r] * self.fact_inverse[r]
    }
    fn nHr(&self, n: usize, r: usize) -> ModInt {
        self.nCr(n + r - 1, r)
    }
}

type Mod = ModInt;
#[derive(Debug, Clone, Copy, Default)]
struct ModInt {
    value: usize,
}

impl ModInt {
    fn new(n: usize) -> Self {
        ModInt { value: n % MOD }
    }
    fn zero() -> Self {
        ModInt { value: 0 }
    }
    fn one() -> Self {
        ModInt { value: 1 }
    }
    fn value(&self) -> usize {
        self.value
    }
    fn pow(&self, n: usize) -> Self {
        let mut p = *self;
        let mut ret = ModInt::one();
        let mut nn = n;
        while nn > 0 {
            if nn & 1 == 1 {
                ret *= p;
            }
            p *= p;
            nn >>= 1;
        }
        ret
    }
    fn inv(&self) -> Self {
        fn ext_gcd(a: usize, b: usize) -> (isize, isize, usize) {
            if a == 0 {
                return (0, 1, b);
            }
            let (x, y, g) = ext_gcd(b % a, a);
            (y - b as isize / a as isize * x, x, g)
        }

        ModInt::new((ext_gcd(self.value, MOD).0 + MOD as isize) as usize)
    }
}

impl std::ops::Add for ModInt {
    type Output = ModInt;
    fn add(self, other: Self) -> Self {
        ModInt::new(self.value + other.value)
    }
}

impl std::ops::Sub for ModInt {
    type Output = ModInt;
    fn sub(self, other: Self) -> Self {
        ModInt::new(MOD + self.value - other.value)
    }
}

impl std::ops::Mul for ModInt {
    type Output = ModInt;
    fn mul(self, other: Self) -> Self {
        ModInt::new(self.value * other.value)
    }
}

#[allow(clippy::suspicious_arithmetic_impl)]
impl std::ops::Div for ModInt {
    type Output = ModInt;
    fn div(self, other: Self) -> Self {
        self * other.inv()
    }
}

impl std::ops::AddAssign for ModInt {
    fn add_assign(&mut self, other: Self) {
        *self = *self + other;
    }
}

impl std::ops::SubAssign for ModInt {
    fn sub_assign(&mut self, other: Self) {
        *self = *self - other;
    }
}

impl std::ops::MulAssign for ModInt {
    fn mul_assign(&mut self, other: Self) {
        *self = *self * other;
    }
}

impl std::ops::DivAssign for ModInt {
    fn div_assign(&mut self, other: Self) {
        *self = *self / other;
    }
}

trait Bound<T> {
    fn lower_bound(&self, x: &T) -> usize;
    fn upper_bound(&self, x: &T) -> usize;
}

impl<T: PartialOrd> Bound<T> for [T] {
    fn lower_bound(&self, x: &T) -> usize {
        let (mut low, mut high) = (0, self.len());
        while low + 1 < high {
            let mid = (low + high) / 2;
            if self[mid] < *x {
                low = mid;
            } else {
                high = mid;
            }
        }
        if self[low] < *x {
            low + 1
        } else {
            low
        }
    }

    fn upper_bound(&self, x: &T) -> usize {
        let (mut low, mut high) = (0, self.len());
        while low + 1 < high {
            let mid = (low + high) / 2;
            if self[mid] <= *x {
                low = mid;
            } else {
                high = mid;
            }
        }
        if self[low] <= *x {
            low + 1
        } else {
            low
        }
    }
}

#[macro_export]
macro_rules! max {
    ($x: expr) => ($x);
    ($x: expr, $( $y: expr ),+) => {
        std::cmp::max($x, max!($( $y ),+))
    }
}
#[macro_export]
macro_rules! min {
    ($x: expr) => ($x);
    ($x: expr, $( $y: expr ),+) => {
        std::cmp::min($x, min!($( $y ),+))
    }
}

fn main() {
    std::thread::Builder::new()
        .stack_size(128 * 1024 * 1024)
        .spawn(|| Solver::default().solve())
        .unwrap()
        .join()
        .unwrap();
}

#[macro_export]
macro_rules! input {
    () => {};
    (mut $var:ident: $t:tt, $($rest:tt)*) => {
        let mut $var = __input_inner!($t);
        input!($($rest)*)
    };
    ($var:ident: $t:tt, $($rest:tt)*) => {
        let $var = __input_inner!($t);
        input!($($rest)*)
    };
    (mut $var:ident: $t:tt) => {
        let mut $var = __input_inner!($t);
    };
    ($var:ident: $t:tt) => {
        let $var = __input_inner!($t);
    };
}

#[macro_export]
macro_rules! __input_inner {
    (($($t:tt),*)) => {
        ($(__input_inner!($t)),*)
    };
    ([$t:tt; $n:expr]) => {
        (0..$n).map(|_| __input_inner!($t)).collect::<Vec<_>>()
    };
    ([$t:tt]) => {{
        let n = __input_inner!(usize);
        (0..n).map(|_| __input_inner!($t)).collect::<Vec<_>>()
    }};
    (chars) => {
        __input_inner!(String).chars().collect::<Vec<_>>()
    };
    (bytes) => {
        __input_inner!(String).into_bytes()
    };
    (usize1) => {
        __input_inner!(usize) - 1
    };
    ($t:ty) => {
        $crate::read::<$t>()
    };
}

#[macro_export]
macro_rules! println {
    () => {
        $crate::write(|w| {
            use std::io::Write;
            std::writeln!(w).unwrap()
        })
    };
    ($($arg:tt)*) => {
        $crate::write(|w| {
            use std::io::Write;
            std::writeln!(w, $($arg)*).unwrap()
        })
    };
}

#[macro_export]
macro_rules! print {
    ($($arg:tt)*) => {
        $crate::write(|w| {
            use std::io::Write;
            std::write!(w, $($arg)*).unwrap()
        })
    };
}

#[macro_export]
macro_rules! flush {
    () => {
        $crate::write(|w| {
            use std::io::Write;
            w.flush().unwrap()
        })
    };
}

pub fn read<T>() -> T
where
    T: std::str::FromStr,
    T::Err: std::fmt::Debug,
{
    use std::cell::RefCell;
    use std::io::*;

    thread_local! {
        pub static STDIN: RefCell<StdinLock<'static>> = RefCell::new(stdin().lock());
    }

    STDIN.with(|r| {
        let mut r = r.borrow_mut();
        let mut s = vec![];
        loop {
            let buf = r.fill_buf().unwrap();
            if buf.is_empty() {
                break;
            }
            if let Some(i) = buf.iter().position(u8::is_ascii_whitespace) {
                s.extend_from_slice(&buf[..i]);
                r.consume(i + 1);
                if !s.is_empty() {
                    break;
                }
            } else {
                s.extend_from_slice(buf);
                let n = buf.len();
                r.consume(n);
            }
        }
        std::str::from_utf8(&s).unwrap().parse().unwrap()
    })
}

pub fn write<F>(f: F)
where
    F: FnOnce(&mut std::io::BufWriter<std::io::StdoutLock>),
{
    use std::cell::RefCell;
    use std::io::*;

    thread_local! {
        pub static STDOUT: RefCell<BufWriter<StdoutLock<'static>>> =
            RefCell::new(BufWriter::new(stdout().lock()));
    }

    STDOUT.with(|w| f(&mut w.borrow_mut()))
}