#include<fstream.h>
int main()
 {
  long int a,b,k,c,nrcif,nrprob,nrpos,x,nr,cif;
  double prob;
  ifstream fin("cifre.in");
  ofstream fout("cifre.out");
  fin>>a;
  fin>>b;
  fin>>c;
  fin>>k;
  nrprob=0;
  for(nr=a;nr<=b;nr++)
   {
    x=nr;
    nrcif=0;
    while(x!=0)
     {
      cif=x%10;
      if(cif==c) nrcif=nrcif+1;
      x=x/10;
     }
    if(nrcif>=k) nrprob=nrprob+1;
   }
  nrpos=b-a+1;
  prob=(double)nrprob/nrpos;
  fout<<prob<<'\n';
  fin.close();
  fout.close();
  return 0;
 }