program cifra_III;
var a,b,c,k,cop,i,n:longint;
    p:real;
    f,g:text;
    cond:boolean;
begin
assign (f,'cifre.in'); reset (f);
assign (g,'cifre.out'); rewrite (g);
read (f,a,b,c,k);
n:=0;
for i:=a to b do begin cop:=i; cond:=false;
                       while (cop>0) and (cond <>true) do begin if (cop mod 10=c) then begin n:=n+1; cond:=true; end;
                                                                cop:=cop div 10;

                                        end;
                 end;

p:=n/(b-a);
write (g,p:4:4);
close (f);
close (g);
end.