import java.io.*;
import java.util.*;

public class Main{
    public static BufferedReader stdIn = new BufferedReader(new InputStreamReader(System.in));
    public static PrintWriter stdOut = new PrintWriter(new BufferedWriter(new OutputStreamWriter(System.out)));
    public static final int N = 2010;
    public static int[] dist = new int[N * N];
    public static int[] h = new int[N * N];
    public static int[] e = new int[2 * N];
    public static int[] ne = new int[2 * N];
    public static int n, m, idex;
    public static void add(int a, int b) {
        e[idex] = b; ne[idex] = h[a]; h[a] = idex++;
    }
    public static void main(String[] args) throws IOException{
        String[] sp = stdIn.readLine().split(" ");
        n = Integer.parseInt(sp[0]); m = Integer.parseInt(sp[1]);
        Arrays.fill(dist, Integer.MAX_VALUE);
        Arrays.fill(h, -1);
        dist[n * n - 1] = 0;
        while(m-- > 0) {
            sp = stdIn.readLine().split(" ");
            int x1 = Integer.parseInt(sp[0]); int y1 = Integer.parseInt(sp[1]);
            int x2 = Integer.parseInt(sp[2]); int y2 = Integer.parseInt(sp[3]);
            x1--; y1--; x2--; y2--;
            int c1 = x1 * n + y1; int c2 = x2  * n + y2;
            add(c1, c2); add(c2, c1);
        }
        Queue<Integer> q = new LinkedList<Integer>();
        q.add(n * n - 1);
        while(!q.isEmpty()) {
            int c = q.poll();
            int x = c / n; int y = c % n;
            for(int i = h[c]; i != -1; i = ne[i]) {
                int j = e[i];
                if(dist[j] > dist[c] + 1) {
                    dist[j] = dist[c] + 1;
                    q.add(j);
                }
            }
            int[] dx = {1,-1,0,0}; int[] dy = {0,0,1,-1};
            for(int i = 0; i < 4; i++) {
                int a = x + dx[i]; int b = y + dy[i];
                if(a >= 0 && a < n && b >= 0 && b < n) {
                    int j = a * n + b;
                    if(dist[j] > dist[c] + 1) {
                        dist[j] = dist[c] + 1;
                        q.add(j);
                    }
                }
            }
        }
        double sum = 0;
        for(int i = 0; i < n * n; i++) {
            sum += dist[i];
        }
        stdOut.println(String.format("%.2f", sum / (n * n)));
        stdOut.flush();
    }
}