import java.io.ByteArrayInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.ArrayDeque;
import java.util.Arrays;
import java.util.InputMismatchException;
import java.util.Queue;

public class Main {
	static InputStream is;
	static PrintWriter out;
	static String INPUT = "";
	
	static void solve()
	{
		int n = ni();
		int m = ni();
		boolean[][] g = new boolean[n][n];
		for(int i = 0;i < n;i++){
			Arrays.fill(g[i], true);
			g[i][i] = false;
		}
		for(int i = 0;i < m;i++){
			int x = ni()-1, y = ni()-1;
			g[x][y] = g[y][x] = false;
		}
		
		int[] color = new int[n];
		int[] pos = new int[n];
		int[] neg = new int[n];
		Queue<Integer> q = new ArrayDeque<>();
		for(int i = 0;i < n;i++){
			if(color[i] == 0){
				color[i] = 1;
				q.add(i);
				pos[i]++;
				while(!q.isEmpty()){
					int cur = q.poll();
					for(int j = 0;j < n;j++){
						if(g[cur][j]){
							if(color[j] == 0){
								color[j] = -color[cur];
								if(color[j] < 0){
									neg[i]++;
								}else{
									pos[i]++;
								}
								q.add(j);
							}else if(color[j] == color[cur]){
								out.println(-1);
								return;
							}
						}
					}
				}
			}
		}
		
		boolean[] dp = new boolean[n+1];
		dp[0] = true;
		for(int i = 0;i < n;i++){
			if(pos[i] > 0 || neg[i] > 0){
				boolean[] ndp = new boolean[n+1];
				for(int j = 0;j <= n;j++){
					if(dp[j]){
						ndp[j+neg[i]] = true;
						ndp[j+pos[i]] = true;
					}
				}
				dp = ndp;
			}
		}
		for(int i = n/2;i >= 0;i--){
			if(dp[i]){
				out.println(i*(i-1)/2 + (n-i)*(n-i-1)/2);
				return;
			}
		}
		throw new RuntimeException();
	}
	
	public static void main(String[] args) throws Exception
	{
		long S = System.currentTimeMillis();
		is = INPUT.isEmpty() ? System.in : new ByteArrayInputStream(INPUT.getBytes());
		out = new PrintWriter(System.out);
		
		solve();
		out.flush();
		long G = System.currentTimeMillis();
		tr(G-S+"ms");
	}
	
	private static boolean eof()
	{
		if(lenbuf == -1)return true;
		int lptr = ptrbuf;
		while(lptr < lenbuf)if(!isSpaceChar(inbuf[lptr++]))return false;
		
		try {
			is.mark(1000);
			while(true){
				int b = is.read();
				if(b == -1){
					is.reset();
					return true;
				}else if(!isSpaceChar(b)){
					is.reset();
					return false;
				}
			}
		} catch (IOException e) {
			return true;
		}
	}
	
	private static byte[] inbuf = new byte[1024];
	static int lenbuf = 0, ptrbuf = 0;
	
	private static int readByte()
	{
		if(lenbuf == -1)throw new InputMismatchException();
		if(ptrbuf >= lenbuf){
			ptrbuf = 0;
			try { lenbuf = is.read(inbuf); } catch (IOException e) { throw new InputMismatchException(); }
			if(lenbuf <= 0)return -1;
		}
		return inbuf[ptrbuf++];
	}
	
	private static boolean isSpaceChar(int c) { return !(c >= 33 && c <= 126); }
//	private static boolean isSpaceChar(int c) { return !(c >= 32 && c <= 126); }
	private static int skip() { int b; while((b = readByte()) != -1 && isSpaceChar(b)); return b; }
	
	private static double nd() { return Double.parseDouble(ns()); }
	private static char nc() { return (char)skip(); }
	
	private static String ns()
	{
		int b = skip();
		StringBuilder sb = new StringBuilder();
		while(!(isSpaceChar(b))){
			sb.appendCodePoint(b);
			b = readByte();
		}
		return sb.toString();
	}
	
	private static char[] ns(int n)
	{
		char[] buf = new char[n];
		int b = skip(), p = 0;
		while(p < n && !(isSpaceChar(b))){
			buf[p++] = (char)b;
			b = readByte();
		}
		return n == p ? buf : Arrays.copyOf(buf, p);
	}
	
	private static char[][] nm(int n, int m)
	{
		char[][] map = new char[n][];
		for(int i = 0;i < n;i++)map[i] = ns(m);
		return map;
	}
	
	private static int[] na(int n)
	{
		int[] a = new int[n];
		for(int i = 0;i < n;i++)a[i] = ni();
		return a;
	}
	
	private static int ni()
	{
		int num = 0, b;
		boolean minus = false;
		while((b = readByte()) != -1 && !((b >= '0' && b <= '9') || b == '-'));
		if(b == '-'){
			minus = true;
			b = readByte();
		}
		
		while(true){
			if(b >= '0' && b <= '9'){
				num = num * 10 + (b - '0');
			}else{
				return minus ? -num : num;
			}
			b = readByte();
		}
	}
	
	private static long nl()
	{
		long num = 0;
		int b;
		boolean minus = false;
		while((b = readByte()) != -1 && !((b >= '0' && b <= '9') || b == '-'));
		if(b == '-'){
			minus = true;
			b = readByte();
		}
		
		while(true){
			if(b >= '0' && b <= '9'){
				num = num * 10 + (b - '0');
			}else{
				return minus ? -num : num;
			}
			b = readByte();
		}
	}
	
	private static void tr(Object... o) { if(INPUT.length() != 0)System.out.println(Arrays.deepToString(o)); }
}


Measured memory_bytes: 22768