// what is matter? never mind. 
//#pragma GCC optimize("Ofast")
//#pragma GCC optimize("unroll-loops")
//#pragma GCC target("sse,sse2,sse3,sse4,popcnt,abm,mmx,avx,avx2")
#include<bits/stdc++.h>
#define For(i,a,b) for(int i=(a);i<=(b);++i)
#define Rep(i,a,b) for(int i=(a);i>=(b);--i)
#define ll long long
//#define ull unsigned long long
//#define int long long
#define SZ(x) ((int)((x).size()))
#define ALL(x) (x).begin(),(x).end()
using namespace std;
inline int read()
{
	char c=getchar();int x=0;bool f=0;
	for(;!isdigit(c);c=getchar())f^=!(c^45);
	for(;isdigit(c);c=getchar())x=(x<<1)+(x<<3)+(c^48);
	return f?-x:x;
}

#define fi first
#define se second
#define pb push_back
#define mkp make_pair
typedef pair<int,int>pii;
typedef vector<int>vi;

typedef long double db;
const db eps=1e-8,pi=3.14159265358979323846;
int sgn(db x){return x<-eps?-1:x>eps;}
int cmp(db a,db b){return sgn(a-b);}

struct P{
	db x,y;
	P(db x=0,db y=0):x(x),y(y){}
	P&operator +=(P o){return x+=o.x,y+=o.y,*this;}
	P&operator -=(P o){return x-=o.x,y-=o.y,*this;}
	P&operator *=(db o){return x*=o,y*=o,*this;}
	P&operator /=(db o){return x/=o,y/=o,*this;}
	friend P operator +(P a,P b){return a+=b;}
	friend P operator -(P a,P b){return a-=b;}
	friend P operator *(P a,db b){return a*=b;}
	friend P operator /(P a,db b){return a/=b;}
	friend bool operator <(P a,P b){return fabs(a.x-b.x)<eps?a.y<b.y:a.x<b.x;}
	friend bool operator ==(P a,P b){return cmp(a.x,b.x)==0 && cmp(a.y,b.y)==0;}
	friend bool operator !=(P a,P b){return !(a==b);}
	friend db operator %(P a,P b){return a.x*b.x+a.y*b.y;} // dot
	friend db operator *(P a,P b){return a.x*b.y-a.y*b.x;} // cross
	
	P rot(db o){
		db s=sin(o),c=cos(o);
		return P(x*c-y*s,x*s+y*c);
	}
	P rot90(){return P(-y,x);}
	db ang(){return atan2(y,x);}
	db len(){return sqrt(x*x+y*y);}
	db len2(){return x*x+y*y;}
	
	int half(){return sgn(y)==1||(sgn(y)==0&&sgn(x)>=0);}
	P unit(){return ((*this))/len();}
	
	void read(){cin>>x>>y;}
	void out(){cout<<"("<<x<<","<<y<<")"<<endl;}
};
bool cmp_dir(P a,P b){
	if(a.half()!=b.half())return a.half()<b.half();
	return sgn(a*b)>0;
}

db dis(P a,P b){return (a-b).len();}
db cross(P a,P b,P c){
	// (a->b)*(a->c)
	return (b.x-a.x)*(c.y-a.y)-(b.y-a.y)*(c.x-a.x);
}
int cmp3(P a,P b,P c){
	return sgn(cross(a,b,c));
}

bool in_tri(P a,P b,P c,P p){
	if(cmp3(a,b,c)<0) swap(b,c);
	return cmp3(a,b,p)>=0 && cmp3(b,c,p)>=0 && cmp3(c,a,p)>=0;
}
db area_tri(P a,P b,P c){
	return fabs(cross(a,b,c))/2;
}

bool paral(P p1,P p2,P q1,P q2){
	// is parallel
	return sgn((p2-p1)*(q2-q1))==0;
}
P interl(P p1,P p2,P q1,P q2){
	// intersect point
	db s1=cross(q1,q2,p1),s2=-cross(q1,q2,p2);
	return (p1*s2+p2*s1)/(s1+s2);
}

bool inter(db l1,db r1,db l2,db r2){
	if(l1>r1)swap(l1,r1); if(l2>r2)swap(l2,r2);
	return !(cmp(r1,l2)==-1||cmp(r2,l1)==-1);
}
bool ismid(db a,db m,db b){
	return sgn(a-m)==0||sgn(b-m)==0||((a<m)!=(b<m));
}
bool ismid(P a,P m,P b){
	return ismid(a.x,m.x,b.x)&&ismid(a.y,m.y,b.y);
}

bool isseg(P p1,P p2,P q1,P q2){
	return inter(p1.x,p2.x,q1.x,q2.x) && inter(p1.y,p2.y,q1.y,q2.y) &&
	cmp3(p1,p2,q1)*cmp3(p1,p2,q2)<=0 && cmp3(q1,q2,p1)*cmp3(q1,q2,p2)<=0;
}
bool isseg_strict(P p1,P p2,P q1,P q2){
	return cmp3(p1,p2,q1)*cmp3(p1,p2,q2)<0 && cmp3(q1,q2,p1)*cmp3(q1,q2,p2)<0;
}

struct L{
	P a,b;
	L(P aa=P(0,0),P bb=P(0,0)){a=aa,b=bb;}
	bool in(P p){return sgn((b-a)*(p-a))>0;}
	int in_sgn(P p){return sgn((b-a)*(p-a));}
	P dir(){return b-a;}
	bool onl(P p){
		return cmp3(a,b,p)==0;
	}
	bool onseg(P p){
		return onl(p)&&ismid(a,p,b);
	}
	bool onseg_strict(P p){
		return onl(p)&&sgn((p-a)%(a-b))*sgn((p-b)%(a-b))<0;
	}
	void out(){cout<<"("<<a.x<<","<<a.y<<")---("<<b.x<<","<<b.y<<")\n";}
};

bool isseg(L a,L b){
	return isseg(a.a,a.b,b.a,b.b);
}
bool paral(L a,L b){
	// is parallel
	return paral(a.a,a.b,b.a,b.b);
}
P operator &(L a,L b){
	return interl(a.a,a.b,b.a,b.b);
}
bool samedir(L a,L b){
	return paral(a,b) && sgn(a.dir()%b.dir())==1;
}
bool operator <(L a,L b){
	if(samedir(a,b)) return b.in(a.a);
	return cmp_dir(a.dir(),b.dir());
}

P proj(L a,P b){
	P d=a.dir();
	return a.a+d*((d%(b-a.a))/d.len2());
}
P reflect(L a,P b){
	return proj(a,b)*2-b;
}
db dis(L a,P b){
	db s=abs((b-a.a)*(b-a.b));
	return s/dis(a.a,a.b);
}
db dis_seg(L a,P b){
	if(a.a==a.b) return	dis(a.a,b);
	P h=proj(a,b);
	if(ismid(a.a,h,a.b)) return dis(h,b);
	return min(dis(a.a,b),dis(a.b,b));
}
db dis_ss(L a,L b){
	if(isseg(a,b)) return 0;
	return min(min(dis_seg(a,b.a),dis_seg(a,b.b)),min(dis_seg(b,a.a),dis_seg(b,a.b)));
}

db rad(P a,P b){
	return atan2l(a*b,a%b);
}

// circles
struct cir{
	P o; db r;
	cir(P oo=P(0,0),db rr=0){o=oo,r=rr;}
	bool in(P x){return sgn(dis(x,o)-r)<=0;}
	bool in_strict(P x){return sgn(dis(x,o)-r)<0;}
};
int type(cir a,cir b){
	db d=dis(a.o,b.o);
	if(cmp(d,a.r+b.r)==1)return 4;
	if(cmp(d,a.r+b.r)==0)return 3;
	if(cmp(d,fabs(a.r-b.r))==1)return 2;
	if(cmp(d,fabs(a.r-b.r))==0)return 1;
	return 0;
}
// cir&line
vector<P> operator &(cir a,L b){
	if(cmp(abs(((a.o-b.a)*(b.b-b.a))/dis(b.a,b.b)),a.r)>=0) return {};
	db x=(b.a-a.o)%(b.b-b.a);
	db y=(b.b-b.a).len2();
	db d=x*x-y*((b.a-a.o).len2()-a.r*a.r);
	d=max(d,(db)0.0);
	P m=b.a-(b.b-b.a)*(x/y);
	P dr=(b.b-b.a)*(sqrt(d)/y);
	return {m-dr,m+dr}; //along dir: p1->p2
}

#define maxn 1000005
#define inf 0x3f3f3f3f

int n;
db R;
vector<P>p;

vector<pair<db,db>>vec;
vector<db>rev;
db t[maxn]; int len;

void sausage(P a,P b){
	if(cmp(a.y,R)>=0 && cmp(b.y,R)>=0) return ;
	if(cmp(a.y,-R)<=0 && cmp(b.y,-R)<=0) return ;
	
	P t=(b-a).rot90();
	t/=t.len(),t*=R;
	L my=L(P(min(a.x,b.x)-R*2,0),P(max(a.x,b.x)+R*2,0));
	db ql=1e18,qr=-ql;
	
	if(cmp(max(a.y,b.y),0.0)>0 && cmp(min(a.y,b.y),0.0)<=0 && isseg(my,L(a,b))){
		P ps=(my&L(a,b));
		assert(!isnan(ps.x));
		::t[++len]=ps.x;
	//	cout<<"rev "<<ps.x<<"\n";
		rev.pb(ps.x);
	}
	
	for(auto l1:{L(a-t,b-t),L(a+t,b+t)})
		if(isseg(my,l1)) {
			P ps=(my&l1);
			//cout<<"PS "<<ps.x<<"\n";
			ql=min(ql,ps.x),qr=max(qr,ps.x);
		}
//	cout<<"R: "<<R<<"\n";
	for(auto cr:{cir(a,R),cir(b,R)}){
		vector<P>tmp=(cr&my);
		if(tmp.size()){
			for(auto ps:tmp){
			//	ps.out();
				ql=min(ql,ps.x),qr=max(qr,ps.x);
			}
		}
	}
	if(ql>qr) return;
	vec.pb(mkp(ql,qr));
//	cerr<<"l,r "<<ql<<" "<<qr<<"\n";
	::t[++len]=ql,::t[++len]=qr;
}

int ban[maxn],c[maxn];
signed main()
{
	cin>>n>>R;
	p.resize(n);
	For(i,0,n-1)cin>>p[i].x>>p[i].y;
	For(i,0,n-1)sausage(p[i],p[(i+1)%n]);
	t[++len]=-1e18,t[++len]=1e18;
	sort(t+1,t+len+1); len=unique(t+1,t+len+1)-t-1;
	for(auto [l,r]:vec){
		int ql=lower_bound(t+1,t+len+1,l)-t;
		int qr=lower_bound(t+1,t+len+1,r)-t;
		ban[ql]++,ban[qr]--;
	}
	for(auto x:rev){
		int id=lower_bound(t+1,t+len+1,x)-t;
		c[id]^=1;
	}
	db res=0;
	For(i,1,len){
		ban[i]+=ban[i-1];
		c[i]^=c[i-1];
		//cout<<"i: "<<i<<" "<<ban[i]<<" "<<c[i]<<"\n";
		//cout<<"l,r "<<t[i]<<" "<<t[i+1]<<"\n";
		if(i<len && !ban[i] && c[i]) res+=t[i+1]-t[i];//cout<<"add\n";
	}
	printf("%.14lf\n",(double)res);
	return 0;
}
/*

*/