/*-------------------- Code written by Kabeton --------------------*/
/*--- Contacts: https://vk.com/kabeton and TheKabeton@yandex.ru ---*/

#define _USE_MATH_DEFINES
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <algorithm>
#include <vector>
#include <map>
#include <unordered_map>
#include <set>
#include <bitset>
#include <tuple>
#include <queue>
#include <stack>
#include <valarray>
#include <sstream>
#include <cstring>
#include <numeric>
#include <ctime>
#include <cassert>

using namespace std;

#define endl "\n"
#define re return
#define fi first
#define se second
#define all(a) a.begin(), a.end()

typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
typedef pair<ll, ll> pll;
typedef vector<ll> vll;
typedef vector<vll> vvll;
typedef vector<pll> vpll;
typedef vector<vpll> vvpll;


const ll mod = 1e9 + 7;
const ll inf = 1e15;

struct pt {
    double x, y;
};

struct line {
    double a, b, c;
};

const double EPS = 1e-9;

double det (double a, double b, double c, double d) {
    return a * d - b * c;
}

bool intersect (line m, line n, pt & res) {
    double zn = det (m.a, m.b, n.a, n.b);
    if (abs (zn) < EPS)
        return false;
    res.x = - det (m.c, m.b, n.c, n.b) / zn;
    res.y = - det (m.a, m.c, n.a, n.c) / zn;
    return true;
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);

    ll n, h, x, y, xh, yh;
    long double ans = 0;
    cin >> n >> h;
    vector<pair<ll, ll>> v;

    for (ll i = 0; i < n; ++i) {
        cin >> x >> y;
        v.emplace_back(x, y);
    }

    xh = v[0].first;
    yh = v[0].second + h;

    long double alpha = M_PI + M_PI / 2;
    ll xm = -1;
    ll ym = -1;
    ll q = 0;

    for (ll i = 0; i < n; ++i) {
        long double alpha2;

        ll xc = xh -= v[i].first;
        ll yc = yh -= v[i].second;
        if (xc == 0) {
            if (y > 0) {
                alpha2 = M_PI / 2;
            } else {
                alpha2 = M_PI + M_PI / 2;
            }
        } else {
            alpha2 = atan(yc / xc);
        }

        if ((alpha2 - alpha + 0.0000000001) > 0) {
            if (q == 1) {
                q = 0;
                pt ptt{0, 0};
                double a, b, c;
                a = yh - ym;
                b = xm - xh;
                c = -a * xh - b * yh;
                line line1{a, b, c};

                a = v[i].second - v[i - 1].second;
                b = v[i - 1].first - v[i].first;
                c = -a * v[i].first - b * v[i].second;
                line line2{a, b, c};

                intersect(line1, line2, ptt);

                ans += sqrt((v[i].first - ptt.x) * (v[i].first - ptt.x) +
                                    (v[i].second - ptt.y) * (v[i].second - ptt.y));
            } else {
                alpha = alpha2;
                xm = v[i].first;
                ym = v[i].second;
            }
        } else {
            ans += sqrt((v[i].first - v[i - 1].first) * (v[i].first - v[i - 1].first) +
                    (v[i].second - v[i - 1].second) * (v[i].second - v[i - 1].second));
            q = 1;
        }
    }

    cout << ans;

    re 0;
}