using System;
using System.Linq;
using System.Collections.Generic;
using Debug = System.Diagnostics.Debug;
using StringBuilder = System.Text.StringBuilder;
//using System.Numerics;
//using Point = System.Numerics.Complex;
using Number = System.Int64;
namespace Program
{
    public class Solver
    {
        public void Solve()
        {
            var n = sc.Integer();
            var m = sc.Integer();
            G = Enumerate(n, x => new List<int>());
            c = Enumerate(n, x => -1);
            for (int i = 0; i < m; i++)
            {
                var a = sc.Integer() - 1;
                var b = sc.Integer() - 1;
                G[a].Add(b);
                G[b].Add(a);
            }
            var cnt = new int[n + 1];
            var min = 0;
            for (int i = 0; i < n; i++)
            {
                if (c[i] == -1)
                {
                    var x = new int[2];
                    dfs(i, 0, x);
                    if (x[0] == -1)
                    {
                        IO.Printer.Out.WriteLine(-1);
                        return;
                    }
                    min += x.Min();
                    cnt[x.Max() - x.Min()]++;
                }
            }
            //Debug.WriteLine(string.Join(" ", cnt));
            var dp = new bool[n + 1];
            dp[0] = true;
            var rem = 0;
            for (int i = 1; i <= n; i++)
            {
                if (cnt[i] == 0) continue;
                rem += cnt[i] * i;
                var X = new List<int>();
                var k = cnt[i];
                for (int j = 0; j < 20; j++)
                {
                    if (k > 1 << j) { X.Add((1 << j) * i); k -= 1 << j; }
                    else { X.Add(k * i); break; }
                }
                foreach (var x in X)
                {
                    for (int j = n; j >= 0; j--)
                        if (dp[j] && j + x <= n) dp[j + x] |= dp[j];
                }
            }
            //Debug.WriteLine(string.Join(" ", dp.Select(x => x ? 1 : 0)));
            var max = 0L;
            for (int i = 0; i <= n; i++)
            {
                if (dp[i])
                {
                    max = Math.Max(max, 1L * (min + i) * (min + rem - i) - m);
                }
            }
            IO.Printer.Out.WriteLine(max);

        }
        int[] c;
        List<int>[] G;

        void dfs(int cur, int k, int[] a)
        {
            c[cur] = k;
            a[k]++;
            foreach (var to in G[cur])
            {
                if (c[to] == -1) dfs(to, k ^ 1, a);
                else if (c[to] == k)
                {
                    a[0] = -1;
                    a[1] = -1;
                    return;
                }
            }
        }

        public IO.StreamScanner sc = new IO.StreamScanner(Console.OpenStandardInput());
        static T[] Enumerate<T>(int n, Func<int, T> f) { var a = new T[n]; for (int i = 0; i < n; ++i) a[i] = f(i); return a; }
        static public void Swap<T>(ref T a, ref T b) { var tmp = a; a = b; b = tmp; }
    }
}

#region main
static class Ex
{
    //static public string AsString(this IEnumerable<char> ie) { return new string(System.Linq.Enumerable.ToArray(ie)); }
    //static public string AsJoinedString<T>(this IEnumerable<T> ie, string st = " ") { return string.Join(st, ie); }
    static public void Main()
    {
        var solver = new Program.Solver();
        solver.Solve();
        Program.IO.Printer.Out.Flush();
    }
}
#endregion
#region Ex
namespace Program.IO
{
    using System.IO;
    using System.Text;
    using System.Globalization;
    public class Printer: StreamWriter
    {
        static Printer() { Out = new Printer(Console.OpenStandardOutput()) { AutoFlush = false }; }
        public static Printer Out { get; set; }
        public override IFormatProvider FormatProvider { get { return CultureInfo.InvariantCulture; } }
        public Printer(System.IO.Stream stream) : base(stream, new UTF8Encoding(false, true)) { }
        public Printer(System.IO.Stream stream, Encoding encoding) : base(stream, encoding) { }
        public void Write<T>(string format, T[] source) { base.Write(format, source.OfType<object>().ToArray()); }
        public void WriteLine<T>(string format, T[] source) { base.WriteLine(format, source.OfType<object>().ToArray()); }
    }
    public class StreamScanner
    {
        public StreamScanner(Stream stream) { str = stream; }
        public readonly Stream str;
        private readonly byte[] buf = new byte[1024];
        private int len, ptr;
        public bool isEof = false;
        public bool IsEndOfStream { get { return isEof; } }
        private byte read()
        {
            if (isEof) return 0;
            if (ptr >= len) { ptr = 0; if ((len = str.Read(buf, 0, 1024)) <= 0) { isEof = true; return 0; } }
            return buf[ptr++];
        }
        public char Char() { byte b = 0; do b = read(); while ((b < 33 || 126 < b) && !isEof); return (char)b; }

        public string Scan()
        {
            var sb = new StringBuilder();
            for (var b = Char(); b >= 33 && b <= 126; b = (char)read())
                sb.Append(b);
            return sb.ToString();
        }
        public string ScanLine()
        {
            var sb = new StringBuilder();
            for (var b = Char(); b != '\n'; b = (char)read())
                if (b == 0) break;
                else if (b != '\r') sb.Append(b);
            return sb.ToString();
        }
        public long Long()
        {
            if (isEof) return long.MinValue;
            long ret = 0; byte b = 0; var ng = false;
            do b = read();
            while (b != 0 && b != '-' && (b < '0' || '9' < b));
            if (b == 0) return long.MinValue;
            if (b == '-') { ng = true; b = read(); }
            for (; true; b = read())
            {
                if (b < '0' || '9' < b)
                    return ng ? -ret : ret;
                else ret = ret * 10 + b - '0';
            }
        }
        public int Integer() { return (isEof) ? int.MinValue : (int)Long(); }
        public double Double() { var s = Scan(); return s != "" ? double.Parse(s, CultureInfo.InvariantCulture) : double.NaN; }
        private T[] enumerate<T>(int n, Func<T> f)
        {
            var a = new T[n];
            for (int i = 0; i < n; ++i) a[i] = f();
            return a;
        }

        public char[] Char(int n) { return enumerate(n, Char); }
        public string[] Scan(int n) { return enumerate(n, Scan); }
        public double[] Double(int n) { return enumerate(n, Double); }
        public int[] Integer(int n) { return enumerate(n, Integer); }
        public long[] Long(int n) { return enumerate(n, Long); }
    }
}
#endregion


Measured memory_bytes: 22476