#include<bits/stdc++.h>
#define pii pair<int,int>
#define ll long long
#define cl(x,y) memset(x,y,sizeof(x))
#define ct cerr<<"Time elapsed:"<<1.0*clock()/CLOCKS_PER_SEC<<"s.\n";
#define mp make_pair
#define pb push_back
#define fi first
#define se second
#define all(x) x.begin(),x.end()
#define lson x<<1,l,mid
#define rson x<<1|1,mid+1,r
#define INF 1e18
const int N=1e6+10;
const int mod=1e9+7;
const double inf=1e18;
const double eps=1e-8;
const double pi=acos(-1);
using namespace std;
int n,top;
int sgn(double x)
{
    if(fabs(x) < eps)return 0;
    if(x < 0) return -1;
    else return 1;
}
struct point
{
    double x,y;
    point(){}
    point(double xx,double yy){x=xx;y=yy;}
    point operator +(point b){return point(x+b.x,y+b.y);}
    point operator -(point b){return point(x-b.x,y-b.y);}
    double operator ^(point b){return x*b.y-y*b.x;}//叉乘 
    double operator *(point b){return x*b.x+y*b.y;}//点乘 
    point operator *(double b){return point(x*b,y*b);}//数乘   
    void read(){scanf("%lf%lf",&x,&y);}
}p[N];
struct line
{
    point s,e,l;
    line(){}
    line(point ss,point ee){s = ss;e = ee;l=e-s;}
    double gettan(){return atan2(e.y-s.y,e.x-s.x);}
};
double dis(point a,point b)
{
	return sqrt((a-b)*(a-b));
}
point getcross(line l1,line l2)
{
	double s1=((l2.s-l1.s)^(l2.e-l1.s));
	double s2=((l2.e-l1.e)^(l2.s-l1.e));
	if(s1+s2==0)//平行 
		return point{inf,inf};
	return l1.s+l1.l*(s1/(s1+s2));
}
int main()
{
	int i;
	double h;
	scanf("%d%lf",&n,&h);
	for(i=1;i<=n;i++)
		p[i].read();		
	double sum=0;
	point mx=p[n],eye={p[n].x,p[n].y+h};
	for(i=n-1;i>=1;i--)
	{
		point ju=mx-eye,p1=p[i]-eye,p2=p[i+1]-eye;
		int f1=((ju^p1)<=0),f2=((ju^p2)<=0);
		if(f1 && f2)
			sum+=dis(p[i],p[i+1]);			
		else if(f1 && !f2)
		{
			point g=getcross(line{eye,mx},line{p[i],p[i+1]});
			sum+=dis(g,p[i]);
		}
		if(f1)
			mx=p[i];
	}
	printf("%.9lf\n",sum);
	return 0;
}