#include<fstream.h>
#include<stdio.h>
#include<iomanip.h>

long a, b;
int c, k;

void citire()
{
  ifstream in("cifre.in");
  in>>a>>b>>c>>k;
  in.close();
}

int nrc(long x)
{
  int s=0;
  while (x)
    {
      if(x%10==c) s++;
      x/=10;
    }
   return s;
}

void calcul()
{
  int suma=0, nr;
  float rez;
  long i;
  for (i=a; i<=b; i++)
     if (nrc(i)>=k) suma++;
  nr=b-a+1;
  rez=(float)suma/nr;
  FILE *out=fopen("cifre.out","w");
  fprintf(out,"%5.4f",rez);
  fclose(out);
}

int main()
{
  citire();
  calcul();
  return 0;
}