#include<stdio.h>



int main(){

FILE *in=fopen("cifra.in","rt"),*out=fopen("cifra.out","w+");
long a,b,c,k,i,j,ct=0,t,n;

fscanf(in,"%ld",&a);
fscanf(in,"%ld",&b);
fscanf(in,"%ld",&c);
fscanf(in,"%ld",&k);

for(i=a;i<=b;i++){
 n=i;t=0;
 while(n>0){
  if(n%10==c)t++;
   n=n/10;
 }
 if(t>=k)ct++;
}
fprintf(out,"%f",(float)ct/(b-a+1));



fclose(in);fclose(out);
return 1;
}