#include<fstream.h>
#include<math.h>
int main(){
ifstream fin("cifre.in");
ofstream fout("cifre.out");
long a,b,x;
double z=0;
short  c,k=0,nr=0;
fin>>a>>b>>c>>k;
for (long i=pow(10,k);i<=b;i++){
		    x=i;
		    nr=0;
		    while (x>0){
		     if (x%10==c)
			 nr++;
			 x/=10; }
		if (nr>=k)
		    z++;}
long h=b-a;
  z/=(h+1);
  fout<<z<<"\n";
fin.close();
fout.close();
return 0;
}