var f,g:text;
    a,b,n,m,nr,i,c,y,aux:longint;
    z:real;
    ok:boolean;
begin
assign(f,'cifre.in');reset(F);
assign(g,'cifre.out');rewrite(G);
readln(f,a,b,n,m);
nr:=0;

for i:=a to b do
    begin
    aux:=i;
    c:=0;
    y:=0;
    ok:=false;
    while aux>0 do
    begin
    c:=aux mod 10;
    aux:=aux div 10;
    if c=n then y:=y+1;
    if y=m then begin ok:=true; break;end;
    end;
    if ok=true then nr:=nr+1;
    end;
z:=nr/(b-a+1);
writeln(g,z:2:4);
close(G);
end.