#include <bits/stdc++.h>
using namespace std;
typedef unsigned long long int ulli;
#define F(i, n, m) for (int i = n; i <= m; i++)

inline int read() {
    bool w = 0;
    int s = 0;
    char ch = 0;
    while (!isdigit(ch)) w = ch == '-', ch = getchar();
    while (isdigit(ch)) s = (s << 3) + (s << 1) + (ch ^ 48), ch = getchar();
    return w ? -s : s;
}

inline void write(int x) {
    if (x < 0)
        putchar('-'), x = ~x + 1;
    if (x > 9)
        write(x / 10);
    putchar((x % 10) | 48);
}

double p[105][2];
int r;

double dis(int a, int b) {
    double a1, a2, b1, b2;
    a1 = p[a][0];
    a2 = p[a][1];
    b1 = p[b][0];
    b2 = p[b][1];
    return sqrt((a1 - b1) * (a1 - b1) + (a2 - b2) * (a2 - b2));
}

double rec(int i) {
    double ang, a1, a2, b1, b2;
    a1 = p[i - 1][0] - p[i][0];
    a2 = p[i - 1][1] - p[i][1];
    b1 = p[i + 1][0] - p[i][0];
    b2 = p[i + 1][1] - p[i][1];
    ang = acos((a1 * b1 + a2 * b2) / dis(i - 1, i) / dis(i, i + 1));
    return acos(dis(i - 1, i) / 2 / r) + acos(dis(i, i + 1) / 2 / r) - ang;
}

int main() {
    int n = read();
    double ans = 0;
    r = read();
    F(i, 1, n)
    scanf("%lf %lf", &p[i][0], &p[i][1]);
    p[n + 1][0] = p[1][0];
    p[n + 1][1] = p[1][1];
    F(i, 2, n)
    ans += dis(1, i) * rec(i);
    printf("%.15f", ans);
    return 0;
}