#include <iostream.h>
#include <fstream.h>
int main()
{
 ifstream fin("cifre.in");
 ofstream fout("cifre.out");
 long a,b,i,n;
 double prob;
 int c,k,cif,nr,sol;
 fin>>a;
 fin>>b;
 fin>>c;
 fin>>k;
 sol=0;
 for (i=a; i<=b; i++)
   {
    n=i;
    nr=0;
    do{
       cif=n%10;
       n=n/10;
       if (cif==c) nr=nr+1;
    }while (n>0);
    if (nr==k) sol=sol+1;
   }
 prob=(double)sol/(b-a+1);
 fout.precision(4);
 fout<<prob<<"\n";
 fin.close();
 fout.close();
 return 0;
}