I've tried every tests i could found.
It's all right.
But i keep getting WA.
This is my program:

var t,n,l:integer;
    a:real;
    sz:array[1..1000]of record
                              x:integer;
                              n:real;
                        end;
    x,min,k:real;
    q:boolean;
begin
     readln(n,a);
     fillchar(sz,sizeof(sz),0);
     sz[1].n:=a;
     sz[2].x:=1;
     for t:=3 to n do
         begin
              sz[t].x:=sz[t-1].x*2-sz[t-2].x;
              sz[t].n:=(sz[t-1].n+1)*2-sz[t-2].n;
         end;
     min:=-1;
     for t:=2 to n-1 do
         begin
              x:=(-sz[t].n)/sz[t].x;
              q:=true;
              for l:=2 to n do
                  begin
                       k:=sz[l].x*x+sz[l].n;
                       q:=q and(k>=0);
                  end;
              k:=sz[n].x*x+sz[n].n;
              if ((k<min)or(min=-1))and(q) then
                 min:=k;
         end;
     writeln(min:0:2);
end.

About your code: i don't know, sorry
About problem: there is simple solution O(1), about 12 strings...

So can you tell the O(1) solution and if possible explain it ? Thank you.

My O(n) solution (I used maths)is 15 strings but I don't know there is a O(1) solution.Please explain it.