// test
// https://qoj.ac/submission/467496
#include <bits/stdc++.h>
using namespace std;
typedef long double LD;

const LD eps = 1e-8;
const LD INF = 1e18;
int sgn (LD x) { return x > eps ? 1 : (x < -eps ? -1 : 0); }

#define cp const point &
struct point {
	LD x, y;
	point operator + (cp a) const { return {x + a.x, y + a.y}; }
	point operator - (cp a) const { return {x - a.x, y - a.y}; }
	point operator * (LD a) const { return {x * a, y * a}; }
	point operator / (LD a) const { return {x / a, y / a}; }
	point unit() const {
		LD s = hypot (x, y);
		return {x / s, y / s};
	}
	point rot90() const { return {-y, x}; }
};

LD det (cp a, cp b) { return a.x * b.y - b.x * a.y; }
LD dot (cp a, cp b) { return a.x * b.x + a.y * b.y; }

#define cl const line &
struct line {
	point s, t;
};

point line_inter (cl a, cl b) {
	LD s1 = det (a.t - a.s, b.s - a.s);
	LD s2 = det (a.t - a.s, b.t - a.s);
	return (b.s * s2 - b.t * s1) / (s2 - s1);
}

bool two_side (cp a, cp b, cl c) {
	return (sgn (det (a - c.s, c.t - c.s)) <= 0) + (sgn (det (b - c.s, c.t - c.s)) <= 0) == 1;
}

void work() {

	LD R;
	int n;
	cin >> n >> R;
	R -= 3e-7;

	vector <point> a;
	
	for (int i = 0; i < n; i++) {
		int x, y;
		cin >> x >> y;
		a.push_back({x, y});
	}


	{
		LD area = 0;
		for (int i = 0; i < n; i++) area += det (a[i], a[(i + 1) % n]);
		if (area < 0) {
			reverse(a.begin(), a.end());
		}
	}

	vector < pair <LD, int> > e;

	for (int i = 0; i < n; i++) {
		point u = a[i % n];
		point v = a[(i + 1) % n];
		if (sgn(u.y) <= 0 && sgn(v.y) > 0) {
			e.push_back({line_inter ({u, v}, {{0, 0}, {1, 0}}).x, -1});
		} else if (sgn (u.y) > 0 && sgn (v.y) <= 0) {
			e.push_back({line_inter ({u, v}, {{0, 0}, {1, 0}}).x, 1});
		}
	}

	for (int i = 0; i < n; i++) {
		point u = a[i % n];
		if (abs(u.y) < R) {
			LD dx = sqrt(R * R - u.y * u.y);
			e.push_back({u.x - dx, 2});
			e.push_back({u.x + dx, -2});
		}
		point v = a[(i + 1) % n];
		if (max(u.y, v.y) <= -R) continue;
		if (min(u.y, v.y) >= R) continue;
		point norm = (v - u).rot90().unit();
		point w = v + norm * R;
		point x = u + norm * R;
		if (!sgn(w.y) && !sgn(x.y)) continue;
		vector <line> lines = {
			{u, v},
			{v, w},
			{w, x},
			{x, u}
		};
		vector <LD> inter;
		for (auto l : lines) {
			if (two_side (l.s, l.t, {{0, 0}, {1, 0}})) {
				inter.push_back(line_inter (l, {{0, 0}, {1, 0}}).x);
			}
		}
		sort(inter.begin(), inter.end());
		if (inter.size() && inter.back() - inter.front() > eps) {
			e.push_back({inter.front(), 2});
			e.push_back({inter.back(), -2});
		}
	}

	sort(e.begin(), e.end());
	LD last = -INF, ans = 0;
	int cnt = 0, ban = 0;
	for (auto [x, d] : e) {
		//cerr << "last = " << last << " x = " << x << " " << cnt << " " << ban << endl;
		if (cnt == 1 && !ban) ans += x - last;
		if (abs(d) == 1) {
			cnt += d;
		} else {
			ban += d;
		}
		assert (0 <= cnt && cnt <= 1);
		last = x;
	}
	assert (cnt == 0);
	cout << fixed << setprecision(9) << ans << '\n';
}

int main() {
	ios::sync_with_stdio(false); cin.tie(0);
	int T = 1;
//	cin >> T;
	for (int ca = 1; ca <= T; ca ++) {
		work();
	}
}