#include <stdio.h>

 #include <cstring>

 #include <cstdlib>

 #include <algorithm>

 #include <cmath>

 using namespace std;

 #define lson o<<1

 #define rson o<<1|1

 #define max(a,b) (a)>(b)?(a):(b)

 #define min(a,b) (a)<(b)?(a):(b)

 #define INF 200000000

 #define N 100010

 typedef long long ll;

 long double lg[N];

 void init (){

      lg[]=;

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

        lg[i]=(long double)log(i)+lg[i-];

     }

 }

 int main(){

    int n,k,cs=;

    init();

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

       long double p,ans=;

       int up,down,put=*k;

       if(*k>n-)put=n-;

       long double l0=lg[n-]-lg[n--put];

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

           up=max(i--k,);

           down=max(n-i-k,);

           if(up+down>=put)p=exp(lg[up+down]-lg[up+down-put]-l0);

           else p=;

           ans+=-p;

       }

       printf("Case %d: %.4lf\n",cs++,(double)ans);

    }

    return ;

 }