#include <bits/stdc++.h>
#define _for(i, a, b)  for (int i = (a); i <= (b); i ++ )
#define _all(i, a, b)  for (int i = (a); i >= (b); i -- )
#define int long long
#define ll long long
#define ld long double
using namespace std;
const int N = 1e7 + 5; const ld pi = acos( - 1);
int n, m, t, k; ld tmp, ans1, ans2, inv_m, res, res1, res2;
inline void Add(ld & x, ld y) { x = x + y; }
inline void Mul(ld & x, ld y) { x *= y; }
inline ld pw(ld a, int b) {
	ld res = 1;
	while (b) {
		if (b & 1)  Mul(res, a);
		Mul(a, a), b >>= 1;
	}
	return res;
}
signed main() {
	ios :: sync_with_stdio(false), cin.tie(0), cout.tie(0);
	cin >> n >> m; ld inv_m = 1.0 / m, inv_2 = 0.5;
	if (m == 1) {
		_for (i, - n,n)  if (i)  _for (j, i + 1, n)  if (j)  res += j - i;
		return cout << fixed << setprecision(6) << res << "\n", 0;
	}
	_for (i, 1, 2 * n)  Add(res, 1ll * i * (2 * n - i + 1));
	int l = 1, r = m, mid; Add(ans2, 1ll * res * m );
	while (l <= r) {
		mid = (l + r) >> 1;
		if (0.5 / m * mid <= 1.0 / pi)  t = mid, l = mid + 1;
		else  r = mid - 1;
	}
	_for (i, 1, n)  k = n - i, tmp = 1ll * k * (k + 1)  * inv_2 , Add(res1, 2ll * i * t  * (t + 1)  * inv_m  * k ), Add(res2, 2ll * (m - 1) * tmp ), Add(res2, 4ll * (m - t - 1) * i  * k );
	Add(ans1, 1ll * res1 * m ), Add(ans2, 2ll * res2 * m ), res1 = res2 = 0;
	_for (i, 1, n)  Add(res1, 1ll * i * t  * (t + 1)  * inv_m ), Add(res2, 4ll * (m - t - 1) * i );
	Add(ans1, 1ll * res1 * m ), Add(ans2, 1ll * res2 * m ), cout << fixed << setprecision(6) << ans1 * pi + ans2 << "\n";
	return 0;
}