program ural1066;
var
  n,i:integer;
  a,b,c,l,r,h2:real;
  h1:real;
begin
  readln(n,h1);
  l:=0;r:=maxlongint;
  repeat
    b:=h1;
    h2:=(l+r)/2;c:=h2;
    for i:=3 to n do begin
      a:=b;b:=c;c:=2*(b+1)-a;
      if c<0 then break;
    end;
    if c<0 then l:=h2 else r:=h2;
  until r-l<1e-4;
  writeln(abs(c):0:2);
end.

You should use math to solve this problem

When I inputed 1000 1, my prog gave me 996004.06. So I knew it was the prob of accuration. But when I changed '1e-4' in the last line but two into '1e-10', it worked well on my PC, but WA test #1 on the judge!
  It's even stranger!

You solutions is not correct.You can try these test:
100 100 =>7921.00
10 11 =>32.00
123 456 =>10128.86
111 111 =>9891.00
8 15 =>9.75
5 10 =>0.67
70 50 =>3835.14
1000 1=>996004.00
(It was given by an ACed program)
If you don't know how to solve this problem by maths I can tell you.
I used only 9 var and 12 lines program, O(N).
Goodluck!:)

Subj