#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <ctime>
#include <algorithm>
#include <cstring>
#include <vector>
#include <set>
#include <map>
#include <bitset>
#include <queue>
#include <complex>
#include <cassert>

using namespace std;

#define pb push_back
#define mp make_pair
#define fs first
#define sc second
#define sz(s) int((s).size())
#define eprintf(...) fprintf(stderr, __VA_ARGS__)
#define next _next
#define prev _prev
#define rank _rank

typedef long long ll;
typedef long long llong;
typedef unsigned int uint;
typedef unsigned long long ull;
typedef vector <int> vi;
typedef pair <int, int> pii;
typedef complex <double> tc;

const int inf = int(1e9);
const double eps = 1e-9;
const double pi = 4 * atan(double(1));

long double d[555][555];

int main(){
    //freopen("input.txt", "r", stdin);
    //freopen("output.txt", "w", stdout);
    freopen("runs.in", "r", stdin);
    freopen("runs.out", "w", stdout);
    int n, k;
    scanf("%d %d", &n, &k);
    if(k >= n){
        printf("0\n");
        return 0;
    }
    for(int i = 1; i <= n; i++){
        for(int j = 0; j <= min(i - 1, k); j++){
            if(i == 1 && j == 0){
                d[i][j] = 1;
            }
            else{
                d[i][j] = (j * d[i - 1][j] + 2 * (j >= 1 ? d[i - 1][j - 1] : 0) + (i - j) * (j >= 2 ? d[i - 1][j - 2] : 0)) / i;
            }
        }
    }
    printf("%.9Lf\n", d[n][k]);
    return 0;
}