#include<fstream.h>
#include<stdio.h>
#include<iostream.h>

int main()
{
 long int a,b,c,k,cop,i,n,sol,pow,cond;
 //double p;
 ifstream f("cifre.in");
 ofstream g("cifre.out");
 f>>a>>b>>c>>k;
 sol=0;
 pow=1;
 for (i=1;i<=k-1;i++) pow*=10;
 for (i=a;i<=b;i++) { cop=i;
                      cond=0;
		      n=0;
		      while ((cop>0)&&(cond!=1)&&(cop/pow>=k-n)) {
								 if (cop%10==c) n+=1;
								 if (k==n) { sol+=1;
									     cond=1;
									   }
								 cop/=10;
								}
		    }
 //p=sol/(b-a+1);
 freopen("cifre.out","w",stdout);
 printf("%.4f",(double)sol/(b-a+1));
 f.close();
 g.close();					
 return 0;
}