program alex;
var f:text;
    a,b,c,k,d,nr,i,j:longint;
    v:real;
begin
assign(f,'cifre.in');reset(f);
readln(f,a,b,c,k);
close(f);
nr:=0;
for i:=a to b do
    begin
    d:=0;
    j:=i;
    while j<>0 do
          begin
          if j mod 10=c then d:=d+1;
          j:=j div 10;
          end;
    if d>=k then nr:=nr+1;
    end;
assign(f,'cifre.out');rewrite(f);
v:=nr/(b-a+1);
writeln(f,v:0:4);
close(f);
end.