use std::cmp;

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 graph = vec![vec![false; n]; n];
    for _ in 0..m {
        let a = sc.read::<usize>() - 1;
        let b = sc.read::<usize>() - 1;
        graph[a][b] = true;
        graph[b][a] = true;
    }

    let mut co_graph = vec![vec![true; n]; n];
    for i in 0..n {
        for j in 0..n {
            if graph[i][j] {
                co_graph[i][j] = false;
            }
        }
    }

    let mut color = vec![0; n];
    let mut cur = 1;
    for i in 0..n {
        if color[i] == 0 {
            color[i] = cur;
            if !dfs(i, cur, cur + 1, &mut color, &co_graph) {
                println!("-1");
                return;
            }
            cur += 2;
        }
    }

    let p = cur / 2;
    let mut pairs = vec![(0, 0); p];
    for i in 0..n {
        let pos = (color[i] - 1) / 2;
        if color[i] % 2 == 1 {
            pairs[pos].0 += 1;
        } else {
            pairs[pos].1 += 1;
        }
    }

    let mut dp = vec![vec![false; n + 1]; n + 1];
    dp[0][0] = true;
    for (a, b) in pairs.into_iter() {
        let mut next = vec![vec![false; n + 1]; n + 1];
        for i in 0..n {
            for j in 0..n {
                if !dp[i][j] {
                    continue;
                }
                if i + a <= n && j + b <= n {
                    next[i + a][j + b] = true;
                }
                if i + b <= n && j + a <= n {
                    next[i + b][j + a] = true;
                }
            }
        }
        dp = next;
    }

    let mut ans = 1 << 50;
    for i in 0..(n + 1) {
        for j in 0..(n + 1) {
            if dp[i][j] {
                let sum = if i > 0 { i * (i - 1) / 2 } else { 0 }
                    + if j > 0 { j * (j - 1) / 2 } else { 0 };
                ans = cmp::min(ans, sum);
            }
        }
    }
    println!("{}", ans);
}

fn dfs(
    v: usize,
    red: usize,
    blue: usize,
    color: &mut Vec<usize>,
    co_graph: &Vec<Vec<bool>>,
) -> bool {
    for next in 0..co_graph[v].len() {
        if next == v {
            continue;
        }
        if !co_graph[v][next] {
            continue;
        }
        if color[next] == 0 {
            if color[v] == red {
                color[next] = blue;
            } else {
                color[next] = red;
            }
            if !dfs(next, red, blue, color, co_graph) {
                return false;
            }
        } else {
            if color[v] == color[next] {
                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) -> IO<R, W> {
        IO(r, std::io::BufWriter::new(w))
    }
    pub fn write<S: std::ops::Deref<Target = str>>(&mut self, s: S) {
        use std::io::Write;
        self.1.write(s.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: 6524