#![allow(unused_imports)]
#![allow(dead_code)]
#![allow(unused_must_use)]
use std::io::{stdin, stdout, stderr, Write, BufWriter, BufReader, BufRead};
use std::collections::BinaryHeap;

use util::IterExt;
macro_rules! read_from_line {
    ($inbuf:ident => $($name:ident : $ty:ty),* ) => {
        let mut buf = String::new();
        $inbuf.read_line(&mut buf).unwrap();
        buf.pop();
        let mut words = buf.split_whitespace();
        $(
            let $name:$ty = words.next().unwrap().parse().unwrap();
        )*
    };
}

macro_rules! read_many {
    ($inbuf:ident => $name:ident : $ty:ty) => {
        let $name: Vec<$ty> = strio::read_many_from_line(&mut $inbuf);
    };
}

#[derive(PartialEq, Eq, Debug)]
struct RevOrd<T>(T);

impl<T> PartialOrd for RevOrd<T> where T: PartialOrd {
    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
        other.0.partial_cmp(&self.0)
    }
}

impl<T> Ord for RevOrd<T> where T: Ord {
    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
        other.0.cmp(&self.0)
    }
}

#[derive(Debug)]
struct KV<K, V> {
    k: K,
    v: V
}
impl <K, V> KV<K, V> {
    fn new(k: K, v: V) -> Self {
        KV {k:k, v:v}
    }
}
impl <K, V> PartialEq for KV<K, V> where V: PartialEq {
    fn eq(&self, other: &Self) -> bool {
        self.v.eq(&other.v)
    }
}
impl <K, V> Eq for KV<K, V> where V: Eq { }
impl<K, V> PartialOrd for KV<K, V> where V: PartialOrd {
    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
        self.v.partial_cmp(&other.v)
    }
}
impl<K, V> Ord for KV<K, V> where V: Ord {
    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
        self.v.cmp(&other.v)
    }
}

#[derive(Debug)]
struct UnionFind {
    tree: Vec<usize>,
}

impl UnionFind {
    fn new(n: usize) -> UnionFind {
        UnionFind { 
            tree: (0..n+1).collect()
        }
    }

    fn root(&mut self, i: usize) -> usize {
        let mut r = i;
        let mut stack = vec![];
        while self.tree[r] != r {
            stack.push(r);
            r = self.tree[r];
        }

        for x in stack {
            self.tree[x] = r
        }
        r
    }

    fn unify(&mut self, i: usize, j: usize) {
        let i = self.root(i);
        let j = self.root(j);
        self.tree[std::cmp::max(i, j)] = std::cmp::min(i, j);
    }

}


fn solve() {
    let cout = stdout();
    let mut cout = BufWriter::new(cout.lock());
    let cin = stdin();
    let mut cin = BufReader::new(cin.lock());

    read_from_line!(cin => n: usize, m: usize);
    let es = {
        let mut es = vec![vec![false; n+1]; n+1];
        for i in 1..m+1 {
            read_from_line!(cin => a: usize, b: usize);
            es[a][b] = true;
            es[b][a] = true;
        }
        es
    };
    let mut uf = UnionFind::new(n+1);
    for i in 1..n+1 {
        for j in i..n+1 {
            if !es[i][j] {
                uf.unify(i, j);
            }
        }
    }

    let mut check_bi = vec![None; n+1];
    let mut counts = vec![];
    let mut contradiction = false;
    'mainloop: for i in 1..n+1 {
        if (uf.root(i) != i) { 
            continue;
        }
        let mut stack = vec![i];
        let mut t_count = 1;
        let mut f_count = 0;
        check_bi[i] = Some(true);
        while let Some(v) = stack.pop() {
            let v_sig = check_bi[v].unwrap();
            for u in 1..n+1 {
                if es[v][u] || v == u {
                    continue;
                }
                match check_bi[u] {
                    None => {
                        check_bi[u] = Some(!v_sig);
                        if v_sig {
                            f_count += 1;
                        } else {
                            t_count += 1
                        }
                        stack.push(u);
                    },
                    Some(s) => 
                        if s == !v_sig {
                            continue
                        } else {
                            contradiction = true;
                            break 'mainloop;
                        },
                }
            }
            
        }
        counts.push((t_count, f_count));
    }
    if contradiction {
        writeln!(cout, "-1");
        return;
    }
    let mut possible_sum = vec![false; n+1];
    possible_sum[0] = true;
    for (x, y) in counts {
        let mut next = vec![false; n+1];
        for i in 0..n+1 {
            if (i >= x) {
                next[i] |= possible_sum[i-x];
            }
            if (i >= y) {
                next[i] |= possible_sum[i-y];
            }
        }
        possible_sum = next;
    }
    let ans = (0..n+1).filter(|i| possible_sum[*i]).map(|x| x * (x - 1) / 2 + (n - x) * (n - x - 1) / 2).min().unwrap();
    writeln!(cout, "{}", ans);
    

}

fn main() {
    let stack_size = 100 * 1024 * 1024;
    let thread = std::thread::Builder::new().stack_size(stack_size);
    thread.spawn(|| solve()).unwrap().join().unwrap();
}

mod util {
    use std;
    use std::ops::{Deref, AddAssign};
    use std::iter::{Once, Chain, Scan};
    use std::borrow::Borrow;

    pub trait IterExt where
        Self: Iterator
    {
        fn scan_init<'a, St : 'a, F : 'a> (self, initial_state: St, f: F) -> Box<Iterator<Item=St> + 'a> /*Chain<Once<St>, Scan<Self, St, F>>*/  where
            Self: Sized + 'a,
            F: FnMut(&mut St, Self::Item) -> Option<St>,
            St: Clone
        {
            Box::new(std::iter::once(initial_state.clone()).chain(self.scan(initial_state, f)))
        }

        fn accum<'a, T> (self, init: T) -> Box<Iterator<Item=T> + 'a> where
            Self: Sized + 'a,
            Self::Item: Borrow<T>,
            T: Clone + AddAssign + 'a,
        {
            Box::new(self.scan_init(init, |sum, x| { *sum += x.borrow().clone(); Some(sum.clone()) }))
        }
    }

    impl<T> IterExt for T where
        T: Iterator
    { }

    pub fn test()
    {
        let v:Vec<i32> = vec![1, 4 ,2];
        for x in v.iter().accum(0) {
            println!("{}", x);
        }
        for x in v.iter().accum(0).accum(0) {
            println!("{}", x);
        }

    }

}

mod strio {
    use std::str::FromStr;
    use std;

    pub fn read_from_line<T, R> (r: &mut R) -> T where
        T: FromStr,
        <T as std::str::FromStr>::Err : std::fmt::Debug,
        R: std::io::BufRead,
    {
        let mut buf = String::new();
        r.read_line(&mut buf).unwrap();
        buf.pop();
        buf.parse().unwrap()
    }

    pub fn read_many_from_line<T, R> (r: &mut R) -> Vec<T> where
        T: FromStr,
        <T as std::str::FromStr>::Err : std::fmt::Debug,
        R: std::io::BufRead,
    {
        let mut buf = String::new();
        r.read_line(&mut buf).unwrap();
        buf.pop();
        buf.split_whitespace().map(|x| x.parse().unwrap()).collect()
    }
}


Measured memory_bytes: 8572