#include <cstdio>
#define MAXCIF 10
int c,k;
int cif[MAXCIF+1],fact[MAXCIF+1],p9[MAXCIF+1];
inline int comb(int n,int k){
    return fact[n]/(fact[k]*fact[n-k]);
}
inline int cauta(int x){
   int i,nrcif,rez,f,p,j;
   if(x==-1)
      return 0;
   if(x==0&&c==0&&k<=1)
      return 1;
   nrcif=0;
   while(x>0){
       cif[nrcif++]=x%10;
       x/=10;
   }
   rez=0;
   i=nrcif-1;
   x=0;
   while(i>=0){
       for(p=cif[i]-1;p>=0;p--){
            if(c>0||(c==0&&i<nrcif-1)){
               if(p==c)
                 f=1;
               else
                 f=0;
               j=k-x-f;
               if(j<0)
                 j=0;
               while(j<=i){
                  rez+=comb(i,j)*p9[i-j];
                  j++;
               }
            }
            else
               rez++;
         }
       if(cif[i]==c)
          x++;
       i--;
   }
   return rez;
}
int main(){
   FILE*fi,*fout;
   int a,b,i;
   double rez1,rez2;
   fi=fopen("cifre.in" ,"r");
   fout=fopen("cifre.out" ,"w");
   fscanf(fi,"%d%d%d%d" ,&a,&b,&c,&k);
   fact[0]=1;
   p9[0]=1;
   for(i=1;i<=MAXCIF;i++){
      fact[i]=fact[i-1]*i;
      p9[i]=p9[i-1]*9;
   }
   rez1=cauta(b+1)-cauta(a-1);
   rez2=b-a+1;
   fprintf(fout,"%.4lf" ,rez1/rez2);
   fclose(fi);
   fclose(fout);
   return 0;
}