#include<fstream.h>
#include<math.h>
int main()
   {
   ifstream fin("cifre.in");
   ofstream fout("cifre.out");
   long int A,B;
   int c,k,i,j,x,y;
   double p;
   do {
      fout<<"Dati nr. A ";
      fin>>A;
      fout<<"Dati nr. B ";
      fin>>B;
      }while(0<=A&&A<B&&B<=1000000000);
   do {
      fout<<"Dati cifra c ";
      fin>>c;
      fout<<"Dati cifra k ";
      fin>>k;
      }while(0<=c&&c<=9&&0<=k&&k<=9);
   i=A;
   y=0;
   while(i<=B)
      {
      j=i;
      x=0;
      while(j>0)
	{
	if(j%10==c)x=x+1;
	j=j/10;
	}
      if(x>=k)y=y+1;
      i=i+1;
	}
   fout.precision(4);
   p=y/(B-A);
   fout<<p<<"\n";
   fin.close();
   fout.close();
return 0;
   }