#include<map>

#include<set>

#include<ctime>

#include<cmath>

#include<queue>

#include<string>

#include<vector>

#include<cstdio>

#include<cstring>

#include<iostream>

#include<algorithm>

#include<functional>

using namespace std;

#define ms(x,y) memset(x,y,sizeof(x))

#define rep(i,j,k) for(int i=j;i<=k;i++)

#define per(i,j,k) for(int i=j;i>=k;i--)

#define loop(i,j,k) for (int i=j;i!=-1;i=k[i])

#define inone(x) scanf("%d",&x)

#define intwo(x,y) scanf("%d%d",&x,&y)

#define inthr(x,y,z) scanf("%d%d%d",&x,&y,&z)

#define infou(x,y,z,p) scanf("%d%d%d%d",&x,&y,&z,&p)

#define lson x<<1,l,mid

#define rson x<<1|1,mid+1,r

#define mp(i,j) make_pair(i,j)

#define ft first

#define sd second

typedef long long LL;

typedef pair<int, int> pii;

const int low(int x) { return x&-x; }

const int INF = 0x7FFFFFFF;

const int mod = 1e9 + ;

const int N = 1e6 + ;

const int M = 1e4 + ;

const double eps = 1e-;

double dp[][];

double c[][];

int n,k;

void init()

{

    c[][]=;

    for(int i=;i<=;i++)

    {

        c[i][]=;

        c[i][i]=;

        for(int j=;j<=;j++)

        {

            c[i][j] = c[i-][j] + c[i-][j-];

        }

    }

}

int main()

{

    init();

    while(~scanf("%d%d",&n,&k))

    {

        memset(dp,,sizeof dp);

        for(int i=;i<=n-k;i++) dp[i][]=1.0*i;

        for(int i=;i<=n-k;i++)

        {

            for(int j=;j<=k;j++)

            {

                if(j==) continue;

                int pn = i-j;

                int pk = j;

                for(int p=;p<=pn+pk;p++)

                {

                    double sum=;

                    for(int L=;L<=pn;L++)

                    {

                        int R = p-L;

                        if(R<) break;

                        if(pk<R) continue;

                        sum=sum+dp[pn-L][pk-R]*c[pn][L]*c[pk][R]/c[pn+pk][p];

                    }

                    dp[i][j]=max(sum,dp[i][j]);

                }

            }

        }

        printf("%.10f\n",dp[n-k][k]);

    }

    return ;

}