var f,g:text;
    a,b,k,c,j,i,l,aux,s:longint;
    p,pr:real;
begin
assign(f,'cifre.in');reset(f);
assign(g,'cifre.out');rewrite(g);
readln(f,a,b,c,k);
s:=0;
p:=b-a+1;
for i:=a to b do
    begin
    aux:=i;
    l:=0;
    while aux>0 do
          begin
          if aux mod 10=c then l:=l+1;
          aux:=aux div 10;
          end;
    if l>=k then s:=s+1;
    end;
pr:=p/s;
writeln(g,pr:5:4);
close(f);
close(g);
end.