#include <bits/stdc++.h>
using namespace std;
float r, p, m;
float a[100 * 1000];
int main() {
  cin >> m >> r;
  p = r * sqrt(2);
  a[0] = 2 * r;
  a[1] = 2 * r + p;
  for (int i = (2), _end = (m); i < _end; ++i) {
    a[i] = 2 * p;
    a[i] += 2 * r * (i - 1);
  }
  float sum = 0;
  sum += a[0] * m;
  for (int i = (1), _end = (m); i < _end; ++i) {
    sum += 2 * (m - i) * a[i];
  }
  sum = sum / (m * m);
  printf("%.9f\n", sum);
  return 0;
}
