program cifre;
var f,g:text;
    a,b:0..1000000000;
    c,k,cf:0..9;
    i,o,nr,j:word;
    p:real;



begin
assign(f,'cifre.in'); reset(f);
assign(g,'cifre.out'); rewrite(g);
read(f,a,b,c,k);
nr:=0;
for i:=a to b do
   begin
   o:=i;
   repeat
      cf:=o mod 10;
      if cf=c then begin
                   nr:=nr+1;
                   break;
                   end;
      o:=o div 10;
   until o=0;
   end;
j:=0;
for i:=a to b do
   j:=j+1;
p:=nr/j;
write(g,p:9:4);
close(f);
close(g);
end.