var a,b,i,c,k,numerefavorite,q:longint;
     ch:string; 
     f,g:text;
     rezultat:real;
     
 
 function favnum(nr,kh:longint;ch:string):boolean ; 
  var favourite:boolean;
      s:string; 
      i,num:longint;
      
begin 
  favourite:=false;
  str(nr,s);
  for i:=1 to length(s) do begin 
    if s[i]=ch then inc(num);
    if num>=kh then begin favourite:=true;break;end;
    end; 
   favnum:=favourite; 
 end;
 
 
 BEGIN
  assign(f,'cifre.in');reset(f);
  assign(g,'cifre.out');rewrite(g);
  
  read(f,a,b,c,k); //f
  str(c,ch);
  close(f);
 q:=0;
  for q:=a to b do if(favnum(q,k,ch)) then inc(numerefavorite);
  
  rezultat:=numerefavorite/(b-a+1);
  writeln(g,rezultat:0:4); //g
  close(g);
  END.