#include<stdio.h>
int main()
{
int  a,b,k2,x,c,k,i,cf,m;
float n,contor=0;
FILE *f,*g;
f=fopen("cifre.in","r");
g=fopen("cifre.out","w");
fscanf(f,"%d %d %d %d",&a,&b,&c,&k);
for(i=a;i<=b;i++)
     {x=i;
      k2=0;
      while(x!=0)
	    {cf=x%10;
	    x/=10;
	    if(cf==c)   k2++;}
      if(k2>=k)    contor++;
      }
m=b-a+1;
n=(float)contor/m;
fprintf(g,"%4f ",n);
fclose(f);
fclose(g);
return 0;
}