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