//#include<bits/stdc++.h>
#include <set>
#include <map>
#include <stack>
#include <cmath>
#include <ctime>
#include <queue>
#include <cstdio>
#include <string>
#include <vector>
#include <cstdlib>
#include <cstring>
#include <iostream>
#include <algorithm>
#include <unordered_map>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef pair<ll, int> pli;
typedef pair<int, ll> pil;
#define X first
#define Y second
#define pb push_back
inline ll gcd(ll a, ll b) {
    while (b != 0) {
        ll c = a % b;
        a = b;
        b = c;
    }
    return a < 0 ? -a : a;
}
inline ll lcm(ll a, ll b) { return (a * b) / gcd(a, b); }
inline ll lowbit(ll x) { return x & (-x); }
const double PI = 3.14159265358979323846;
const int inf = 0x3f3f3f3f;
const ll INF = 0x3f3f3f3f3f3f3f3f;
const ll mod = 998244353;
inline char nc() {
    static char buf[1 << 21], *p1 = buf, *p2 = buf;
    return p1 == p2 && (p2 = (p1 = buf) + fread(buf, 1, 1 << 21, stdin), p1 == p2) ? EOF : *p1++;
}
inline ll rd() {
    //#define getchar nc
    ll x = 0, f = 1;
    char ch = getchar();
    while (ch < '0' || ch > '9') {
        if (ch == '-')
            f = -1;
        ch = getchar();
    }
    while (ch >= '0' && ch <= '9') {
        x = (x << 3) + (x << 1) + (ch ^ 48);
        ch = getchar();
    }
    return x * f;
}
const double eps = 1e-8;
const int M = 1e5 + 10;
const int N = 1e6 + 10;
struct Node {
    double x, y;
} p[N];
double r;
double dis(Node a, Node b) { return sqrt((a.x - b.x) * (a.x - b.x) + (a.y - b.y) * (a.y - b.y)); }
double dis2(Node a, Node b) { return (a.x - b.x) * (a.x - b.x) + (a.y - b.y) * (a.y - b.y); }
int main() {
    int n = rd();
    r = rd();
    for (int i = 1; i <= n; i++) p[i].x = rd(), p[i].y = rd();
    p[n + 1] = p[1];
    double ans = 0;
    for (int i = 1; i <= n - 1; i++) {
        double o1 = acos(dis(p[i], p[i + 1]) / (2 * r));
        double fenzi = dis2(p[i], p[i + 1]) + dis2(p[i + 1], p[i + 2]) - dis2(p[i], p[i + 2]);
        double fenmu = 2 * dis(p[i], p[i + 1]) * dis(p[i + 1], p[i + 2]);
        double o = acos(fenzi / fenmu);
        double bet = acos(dis(p[i + 1], p[i + 2]) / (2 * r));
        double ar = o1 + bet - o;
        if (ar >= 0)
            ans += ar * dis(p[1], p[i + 1]);
    }
    printf("%.7f\n", ans);
}