use std::cmp::min;

fn main() {
    let (r, w) = (std::io::stdin(), std::io::stdout());
    let mut sc = IO::new(r.lock(), w.lock());
    let n: usize = sc.read();
    let m: usize = sc.read();
    let mut edge = vec![vec![false; n]; n];
    for _ in 0..m {
        let a = sc.read::<usize>() - 1;
        let b = sc.read::<usize>() - 1;
        edge[a][b] = true;
        edge[b][a] = true;
    }

    let mut inv_graph = vec![vec![]; n];
    for i in 0..n {
        for j in 0..i {
            if edge[i][j] {
                continue;
            }
            inv_graph[i].push(j);
            inv_graph[j].push(i);
        }
    }

    let mut color = vec![0; n];
    let mut t = vec![0; n];
    let mut turn = 0;
    for i in 0..n {
        if color[i] == 0 {
            t[i] = turn;
            color[i] = 1;
            if !dfs(i, &inv_graph, &mut color, &mut t, turn) {
                println!("-1");
                return;
            }
            turn += 1;
        }
    }

    let mut count = vec![[0; 2]; turn];
    for i in 0..n {
        count[t[i]][color[i] - 1] += 1;
    }
    let mut dp = vec![false; n + 1];
    dp[0] = true;
    for e in count.into_iter() {
        let a = e[0];
        let b = e[1];
        let mut next = vec![false; n + 1];
        for cur in 0..(n + 1) {
            if cur + a <= n {
                next[cur + a] |= dp[cur];
            }
            if cur + b <= n {
                next[cur + b] |= dp[cur];
            }
        }
        dp = next;
    }

    let mut ans = n * n;
    for one in 0..(n + 1) {
        if dp[one] {
            let other = n - one;
            let mut cost = 0;
            if one > 0 {
                cost += one * (one - 1) / 2;
            }
            if other > 0 {
                cost += other * (other - 1) / 2;
            }
            ans = min(ans, cost);
        }
    }

    println!("{}", ans);
}

fn dfs(
    v: usize,
    graph: &Vec<Vec<usize>>,
    color: &mut Vec<usize>,
    t: &mut Vec<usize>,
    turn: usize,
) -> bool {
    for &next in graph[v].iter() {
        t[next] = turn;
        if color[next] == 0 {
            color[next] = if color[v] == 1 { 2 } else { 1 };
            if !dfs(next, graph, color, t, turn) {
                return false;
            }
        } else {
            if color[next] == color[v] {
                return false;
            }
        }
    }
    true
}

pub struct IO<R, W: std::io::Write>(R, std::io::BufWriter<W>);

impl<R: std::io::Read, W: std::io::Write> IO<R, W> {
    pub fn new(r: R, w: W) -> Self {
        IO(r, std::io::BufWriter::new(w))
    }
    pub fn write<S: ToString>(&mut self, s: S) {
        use std::io::Write;
        self.1.write(s.to_string().as_bytes()).unwrap();
    }
    pub fn read<T: std::str::FromStr>(&mut self) -> T {
        use std::io::Read;
        let buf = self
            .0
            .by_ref()
            .bytes()
            .map(|b| b.unwrap())
            .skip_while(|&b| b == b' ' || b == b'\n' || b == b'\r' || b == b'\t')
            .take_while(|&b| b != b' ' && b != b'\n' && b != b'\r' && b != b'\t')
            .collect::<Vec<_>>();
        unsafe { std::str::from_utf8_unchecked(&buf) }
            .parse()
            .ok()
            .expect("Parse error.")
    }
    pub fn vec<T: std::str::FromStr>(&mut self, n: usize) -> Vec<T> {
        (0..n).map(|_| self.read()).collect()
    }
    pub fn chars(&mut self) -> Vec<char> {
        self.read::<String>().chars().collect()
    }
}


Measured memory_bytes: 10492