program cifre;
var f,g:text;
    a,b,nr:longint;
    c,k:byte;

procedure citire;
begin
assign (f,'cifre.in'); reset (f);
read (f,a,b,c,k);
close (f);
end;

function verificare (x:longint; y,z:byte):boolean;
var i,v,t:longint;
begin
t:=0;
verificare:=false;
while x>0 do begin
   v:=x mod 10;
   if v=y then inc (t);
   x:=x div 10;
end;
if t>=z then verificare:=true;
end;

procedure calcul;
var i,p:longint;
    m:real;
begin
assign (g,'cifre.out'); rewrite (g);
for i:=a to b do if verificare(i,c,k)=true then inc (nr);
p:=b-a+1;
m:=(nr/p);
write (g,m:5:4);
close (g);
end;

begin
citire;
calcul;
end.