#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int m, r, t, i;
  long double s = 1.4142135623730950488;
  long long int b, f;
  long double d, ans, last, sum = 0;
  long double x, y, z;
  cin >> m >> r;
  x = 2 * r;
  y = 2 * r + (s * r);
  z = 2 * r + (2 * s * r);
  t = (m % 2 == 0) ? (m / 2) : (m / 2 + 1);
  for (i = 1; i <= t; i++) {
    d = x;
    b = (i - 1);
    f = (m - i);
    if (f > 1) {
      d = d + y + z;
      d = d + (f - 2) * z + ((f - 1) * (f - 2) * (x)) / 2;
    }
    if (f == 1) {
      d = y + d;
    }
    if (b > 1) {
      d = d + y + z;
      d = d + (b - 2) * z + ((b - 1) * (b - 2) * (x)) / 2;
    }
    if (b == 1) {
      d = y + d;
    }
    sum = sum + d;
    if (i == t) {
      last = d;
    }
  }
  sum = (m % 2 == 0) ? (2 * sum) : (2 * (sum - last) + last);
  ans = (sum) / (m * m);
  cout << setprecision(16) << ans << endl;
  return 0;
}
