#include <bits/stdc++.h>

using i64 = long long;
using u64 = unsigned long long;
using u32 = unsigned;
using u128 = unsigned __int128;

using F = long double;

int main() {
    std::ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    
    std::cout << std::fixed << std::setprecision(10);
    
    int N, L;
    std::cin >> N >> L;
    
    F ans = 0;
    for (int s = 1; s < (1 << N); s++) {
        int c = 0;
        int k = __builtin_ctz(s);
        int r = k;
        for (int i = k + 1; i < N; i++) {
            if (s >> i & 1) {
                c += std::max(0, i - r - L);
                r = i;
            }
        }
        c += std::max(0, k + N - r - L);
        ans += (__builtin_parity(s) ? 1 : -1) * F(N) / (N - c);
    }
    std::cout << ans << "\n";
    
    return 0;
}