#include <iostream>
#include <cstdio>
#include <cmath>
using namespace std;
int m;
double r, S = 0, temp, d1, c[1000][2];
//计算一号点到二号点的距离
double d(double x1, double y1, double x2, double y2) {
    return sqrt((x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2));
}
//计算角1-2-3的弧度
double jjiao(double x1, double y1, double x2, double y2, double x3, double y3) {
    double a1 = x3 - x2;
    double b1 = y3 - y2;
    double a2 = x1 - x2;
    double b2 = y1 - y2;
    return acos((a1 * a2 + b1 * b2) / (d(x1, y1, x2, y2) * d(x2, y2, x3, y3)));
}
//得到旋转角
double xzjiao(double x1, double y1, double x2, double y2, double x3, double y3, int r) {
    double a = d(x1, y1, x2, y2);  // d1-2
    double b = d(x3, y3, x2, y2);  // d2-3
    double delt = acos(a / (2 * r)) + acos(b / (2 * r)) - jjiao(x1, y1, x2, y2, x3, y3);
    return delt;
}
int main() {
    cin >> m >> r;
    for (int i = 1; i <= m; i++) cin >> c[i][0] >> c[i][1];
    for (int i = 2; i <= m - 1; i++) {  //计算旋转角度
        temp = xzjiao(c[i - 1][0], c[i - 1][1], c[i][0], c[i][1], c[i + 1][0], c[i + 1][1], r);
        //计算旋转中心到1号点的距离
        d1 = d(c[i][0], c[i][1], c[1][0], c[1][1]);
        //计算旋转距离
        S = temp * d1 + S;
    }
    temp = xzjiao(c[m - 1][0], c[m - 1][1], c[m][0], c[m][1], c[1][0], c[1][1], r);  //特殊旋转点m点
    d1 = d(c[m][0], c[m][1], c[1][0], c[1][1]);
    S = S + temp * d1;
    printf("%0.6lf", S);
    return 0;
}