//https://github.com/rust-lang-ja/ac-library-rs

pub mod fenwicktree {
    use std::ops::{Bound, RangeBounds};

    // Reference: https://en.wikipedia.org/wiki/Fenwick_tree
    pub struct FenwickTree<T> {
        n: usize,
        ary: Vec<T>,
        e: T,
    }

    impl<T: Clone + std::ops::AddAssign<T>> FenwickTree<T> {
        pub fn new(n: usize, e: T) -> Self {
            FenwickTree {
                n,
                ary: vec![e.clone(); n],
                e,
            }
        }
        pub fn accum(&self, mut idx: usize) -> T {
            let mut sum = self.e.clone();
            while idx > 0 {
                sum += self.ary[idx - 1].clone();
                idx &= idx - 1;
            }
            sum
        }
        /// performs data[idx] += val;
        pub fn add<U: Clone>(&mut self, mut idx: usize, val: U)
        where
            T: std::ops::AddAssign<U>,
        {
            let n = self.n;
            idx += 1;
            while idx <= n {
                self.ary[idx - 1] += val.clone();
                idx += idx & idx.wrapping_neg();
            }
        }
        /// Returns data[l] + ... + data[r - 1].
        pub fn sum<R>(&self, range: R) -> T
        where
            T: std::ops::Sub<Output = T>,
            R: RangeBounds<usize>,
        {
            let r = match range.end_bound() {
                Bound::Included(r) => r + 1,
                Bound::Excluded(r) => *r,
                Bound::Unbounded => self.n,
            };
            let l = match range.start_bound() {
                Bound::Included(l) => *l,
                Bound::Excluded(l) => l + 1,
                Bound::Unbounded => return self.accum(r),
            };
            self.accum(r) - self.accum(l)
        }
    }

    #[cfg(test)]
    mod tests {
        use super::*;
        use std::ops::Bound::*;

        #[test]
        fn fenwick_tree_works() {
            let mut bit = FenwickTree::new(5, 0i64);
            // [1, 2, 3, 4, 5]
            for i in 0..5 {
                bit.add(i, i as i64 + 1);
            }
            assert_eq!(bit.sum(0..5), 15);
            assert_eq!(bit.sum(0..4), 10);
            assert_eq!(bit.sum(1..3), 5);

            assert_eq!(bit.sum(..), 15);
            assert_eq!(bit.sum(..2), 3);
            assert_eq!(bit.sum(..=2), 6);
            assert_eq!(bit.sum(1..), 14);
            assert_eq!(bit.sum(1..=3), 9);
            assert_eq!(bit.sum((Excluded(0), Included(2))), 5);
        }
    }
}
use fenwicktree::*;

pub mod binary_search {
    type T = usize;

    pub fn binary_search(mut ok: T, mut ng: T, f: impl Fn(T) -> bool) -> T {
        let cont = |ok: T, ng: T| {
            let l = ok.min(ng);
            let r = ok.max(ng);
            l + 1 < r
        };
        while cont(ok, ng) {
            let x = (ok + ng) / 2;
            if f(x) {
                ok = x;
            } else {
                ng = x;
            }
        }
        ok
    }
}

pub mod scanner {

    pub struct Scanner {
        buf: Vec<String>,
    }

    impl Scanner {
        pub fn new() -> Self {
            Self { buf: vec![] }
        }

        pub fn new_from(source: &str) -> Self {
            let source = String::from(source);
            let buf = Self::split(source);
            Self { buf }
        }

        pub fn next<T: std::str::FromStr>(&mut self) -> T {
            loop {
                if let Some(x) = self.buf.pop() {
                    return x.parse().ok().expect("");
                }
                let mut source = String::new();
                std::io::stdin().read_line(&mut source).expect("");
                self.buf = Self::split(source);
            }
        }

        fn split(source: String) -> Vec<String> {
            source
                .split_whitespace()
                .rev()
                .map(String::from)
                .collect::<Vec<_>>()
        }
    }
}

use crate::FenwickTree;
use crate::{binary_search::binary_search, scanner::Scanner};
use std::io::Write;
fn main() {
    let mut scanner = Scanner::new();
    let out = std::io::stdout();
    let mut out = std::io::BufWriter::new(out.lock());
    let t: usize = 1;
    for _ in 0..t {
        solve(&mut scanner, &mut out);
    }
}
fn solve(scanner: &mut Scanner, out: &mut std::io::BufWriter<std::io::StdoutLock>) {
    let n: usize = scanner.next();
    let m: usize = scanner.next();
    let p = (0..n)
        .map(|_| scanner.next::<usize>() - 1)
        .collect::<Vec<_>>();
    let mut count = 0usize;
    let mut bit = FenwickTree::new(n, 0usize);
    for i in 0..n {
        count += p[i] - bit.sum(0..p[i]);
        bit.add(p[i], 1);
    }
    if count == 0 {
        writeln!(out, "0").unwrap();
        return;
    }
    if m % 2 == 0 && count % 2 == 1 {
        writeln!(out, "-1").unwrap();
        return;
    }
    let mut x = binary_search(2e18 as usize / m, 0, |x| count <= x * m);
    if x * m % 2 != count % 2 {
        x += 1;
    }
    writeln!(out, "{}", x * m).unwrap();
}