#include <bits/stdc++.h>
using namespace std;
constexpr double PI = acos(-1), eps = 1e-9;
inline int sgn(double x) {return fabs(x) < eps ? 0 : (x < 0 ? -1 : 1);}
struct Point {
	double x, y;
	friend Point operator - (Point a, Point b) {return {a.x - b.x, a.y - b.y};}
	friend Point operator + (Point a, Point b) {return {a.x + b.x, a.y + b.y};}
	friend Point operator * (Point a, double x) {return {a.x * x, a.y * x};}
	friend Point operator / (Point a, double x) {return {a.x / x, a.y / x};}
	double arg() {
		double r = atan2(y, x);
		if(r < 0) r += 2 * PI;
		return r;
	}
};
struct Line {
	Point s, t;
	Point v() {return t - s;}
};
double cross(Point u, Point v) {
	return u.x * v.y - u.y * v.x;
}
Point inter(Line p, Line q) {	
	double s1 = cross(q.s - p.s, q.t - p.s);
	double s2 = cross(q.t - p.t, q.s - p.t);
	assert(sgn(s1 + s2) != 0);
	return (p.s * s2 + p.t * s1) / (s1 + s2); 
}
bool onleft(Point p, Line q) {
	return sgn(cross(q.t - q.s, p - q.s)) > 0;
}
bool onright(Point p, Line q) {
	return sgn(cross(q.t - q.s, p - q.s)) < 0;
}
int main() {
	//freopen("in.txt", "r", stdin);
	ios::sync_with_stdio(false);
	cin.tie(0), cout.tie(0);
	int n, m;
	cin >> n >> m;
	vector<Point> a(m);
	for(int i = 0; i < m; i ++) {
		int x, y;
		cin >> x >> y;
		a[i].x = x, a[i].y = y;
		// cin >> a[i].x >> a[i].y;
	}
	vector<Line> l;
	l.emplace_back((Line) {0, 0, n, 0});
	l.emplace_back((Line) {n, 0, n, n});
	l.emplace_back((Line) {n, n, 0, n});
	l.emplace_back((Line) {0, n, 0, 0});

	auto calc = [&] (Point u, Point v) {
		vector<Line> cur = l;
		cur.emplace_back((Line) {u, v});
		sort(cur.begin(), cur.end(), [&] (Line u, Line v) {return u.v().arg() < v.v().arg();});
		vector<Line> nxt;
		for(int i = 0; i < cur.size(); i ++) {
			if(!nxt.empty() && sgn(nxt.back().v().arg() - cur[i].v().arg()) == 0){
				if(onleft(cur[i].t, nxt.back())) {
					nxt.pop_back();
				}
			}
			nxt.emplace_back(cur[i]);
		}
		cur = nxt;
		deque<Line> q;
		for(auto u : cur) {
			while(q.size() > 1 && onright(inter(q[q.size() - 2], q.back()), u)) q.pop_back();
			while(q.size() > 1 && onright(inter(q[0], q[1]), u)) q.pop_front();
			q.push_back(u);
		} 
		vector<Point> res;
		for(int i = 0; i < q.size(); i ++) {
			res.emplace_back(inter(q[i], q[(i + 1) % q.size()]));
		}
		double area = 0;
		for(int i = 2; i < res.size(); i ++) area += cross(res[i - 1] - res[0], res[i] - res[0]);
		return area / 2;
	};
	if(m == 1) {
		vector<Point> ret, cur;
		ret.emplace_back((Point) {0, 0});
		ret.emplace_back((Point) {0, n});
		ret.emplace_back((Point) {n, 0});
		ret.emplace_back((Point) {n, n});
		cur = ret;
		for(auto u : ret) {
			for(auto v : l) {
				Point x = inter(v, (Line) {u, a[0]});
				if(x.x > 0 && x.x < n && x.y > 0 && x.y < n) cur.emplace_back(x);
			}
		}
		Point u = a[0];
		sort(cur.begin(), cur.end(), [&] (Point x, Point y) {
			return (x - u) .arg() < (y - u).arg();
		});
		double ans = -1;
		for(int i = 0; i < cur.size(); i ++) {
			Point l = cur[i], r = cur[(i + 1) % cur.size()];
			// cout << u.x << ' ' << u.y << ' ' << v.x << ' ' << v.y << '\n';
			for(int i = 0; i < 200; i ++) {
				Point lmid = (l + l + r) / 3, rmid = (l + r + r) / 3;
				if(calc(u, lmid) < calc(u, rmid)) r = rmid;
				else l = lmid;
			}
			ans = max(ans, 1 - calc(u, l) / n / n);
		}
		cout << fixed << setprecision(10) << ans << '\n';
		return 0;
	}
	double ans = -1;
	for(int i = 0; i < m; i ++) {
		Point u = a[i];
		vector<Point> b = a;
		b.erase(b.begin() + i);
		sort(b.begin(), b.end(), [&] (Point x, Point y) {
			return (x - u).arg() < (y - u).arg();
		});
		int pl = 0;
		for(int i = 0; i < b.size(); i ++) {
			while(pl < b.size() && sgn(cross(b[i] - u, b[pl] - u)) >= 0) pl ++;
			int k = pl - i + 1;
			// cout << u.x << ' ' << u.y << ' ' << b[i].x << ' ' << b[i].y << ' ' << k << '\n';
			ans = max(ans, 1.0 * k / m - calc(u, b[i]) / n / n);
		}
	}
	cout << fixed << setprecision(10) << ans << '\n';
	return 0;
}