macro_rules ! input { ( source = $ s : expr , $ ( $ r : tt ) * ) => { let mut iter = $ s . split_whitespace ( ) ; input_inner ! { iter , $ ( $ r ) * } } ; ( iter = $ iter : ident , $ ( $ r : tt ) * ) => { let s = { use std :: io :: Read ; let mut s = String :: new ( ) ; std :: io :: stdin ( ) . read_to_string ( & mut s ) . unwrap ( ) ; s } ; let mut $ iter = s . split_whitespace ( ) ; input_inner ! { $ iter , $ ( $ r ) * } } ; ( $ ( $ r : tt ) * ) => { let s = { use std :: io :: Read ; let mut s = String :: new ( ) ; std :: io :: stdin ( ) . read_to_string ( & mut s ) . unwrap ( ) ; s } ; let mut iter = s . split_whitespace ( ) ; input_inner ! { iter , $ ( $ r ) * } } ; }
macro_rules ! input_inner { ( $ iter : expr ) => { } ; ( $ iter : expr , ) => { } ; ( $ iter : expr , mut $ var : ident : $ t : tt $ ( $ r : tt ) * ) => { let mut $ var = read_value ! ( $ iter , $ t ) ; input_inner ! { $ iter $ ( $ r ) * } } ; ( $ iter : expr , mut $ var : ident $ ( $ r : tt ) * ) => { input_inner ! { $ iter , mut $ var : usize $ ( $ r ) * } } ; ( $ iter : expr , $ var : ident : $ t : tt $ ( $ r : tt ) * ) => { let $ var = read_value ! ( $ iter , $ t ) ; input_inner ! { $ iter $ ( $ r ) * } } ; ( $ iter : expr , $ var : ident $ ( $ r : tt ) * ) => { input_inner ! { $ iter , $ var : usize $ ( $ r ) * } } ; }
macro_rules ! read_value { ( $ iter : expr , ( $ ( $ t : tt ) ,* ) ) => { ( $ ( read_value ! ( $ iter , $ t ) ) ,* ) } ; ( $ iter : expr , [ $ t : tt ; $ len : expr ] ) => { ( 0 ..$ len ) . map ( | _ | read_value ! ( $ iter , $ t ) ) . collect ::< Vec < _ >> ( ) } ; ( $ iter : expr , { chars : $ base : expr } ) => { read_value ! ( $ iter , String ) . chars ( ) . map ( | c | ( c as u8 - $ base as u8 ) as usize ) . collect ::< Vec < usize >> ( ) } ; ( $ iter : expr , { char : $ base : expr } ) => { read_value ! ( $ iter , { chars : $ base } ) [ 0 ] } ; ( $ iter : expr , chars ) => { read_value ! ( $ iter , String ) . chars ( ) . collect ::< Vec < char >> ( ) } ; ( $ iter : expr , char ) => { read_value ! ( $ iter , chars ) [ 0 ] } ; ( $ iter : expr , usize1 ) => { read_value ! ( $ iter , usize ) - 1 } ; ( $ iter : expr , $ t : ty ) => { $ iter . next ( ) . unwrap ( ) . parse ::<$ t > ( ) . unwrap ( ) } ; }
fn dfs(u: usize, vis: &mut Vec<i64>, g: &Vec<Vec<bool>>) -> Option<i64> {
    let mut x = 1;
    for v in 0..g.len() {
        if u != v && g[u][v] {
            if vis[v] == 0 {
                vis[v] = -vis[u];
                if let Some(y) = dfs(v, vis, g) {
                    x -= y;
                } else {
                    return None;
                }
            } else if vis[u] == vis[v] {
                return None;
            }
        }
    }
    Some(x)
}
fn main() {
    input! { n, m, ab: [(usize1, usize1); m] };
    let mut graph = vec![vec![true; n]; n];
    for i in 0..n {
        graph[i][i] = false;
    }
    for &(a, b) in &ab {
        graph[a][b] = false;
        graph[b][a] = false;
    }
    let mut vis = vec![0; n];
    let mut pr = vec![];
    for i in 0..n {
        if vis[i] == 0 {
            vis[i] = 1;
            if let Some(s) = dfs(i, &mut vis, &graph) {
                pr.push(s.abs() as usize);
            } else {
                println!("{}", -1);
                return;
            }
        }
    }
    pr.sort_by_key(|&i| -(i as i64));
    let mut dp = vec![vec![false; 2 * n + 2]; pr.len() + 1];
    dp[0][n] = true;
    for i in 0..pr.len() {
        let p = pr[i];
        for j in 0..2 * n + 2 {
            if dp[i][j] {
                if j + p <= 2 * n + 1 {
                    dp[i + 1][j + p] = true;
                }
                if j >= p {
                    dp[i + 1][j - p] = true;
                }
            }
        }
    }
    let mut ans = n * (n - 1) / 2;
    for i in 0..2 * n + 2 {
        if dp[pr.len()][i] {
            let d = (i as i64 - n as i64).abs() as usize;
            let a = (n - d) / 2;
            let b = (n + d) / 2;
            let mut x = 0;
            if a > 0 {
                x += a * (a - 1) / 2;
            }
            if b > 0 {
                x += b * (b - 1) / 2;
            }
            if a + b == n {
                ans = std::cmp::min(ans, x);
            }
        }
    }
    println!("{}", ans);
}


Measured memory_bytes: 12540