#include<fstream.h>
#include<iomanip.h>
main()
   {ifstream f("cifre.in");
   ofstream g("cifre.out");
   int a,b,i,c,k,x,d,e,p=0;
   f>>a>>b>>c>>k;
   for(i=a;i<=b;i++)
      {x=i;d=0;
      while(x!=0)  {e=x%10;
		    x=x/10;
		    if(e==c) d++;}
      if(d>=k) p++;}
   p=p/(b-a+1);
   g<<setprecision(4)<<p;
   return 0;}