Don't you think something is tricky with this question?
If A and P are to be natural numbers, then what is the answer if N=0

> Sorry.....see wrongly.......n > 0 but......how come I can't get AC?

program simple;
var n,a,p,i,n2:longint;
begin
  readln(n);
  a:=0; p:=0; n2:=0;
      inc(a);
      repeat
        n2:=n2+a+p;
        inc(p);
      until n2>=n;
  a:=a+(n2-n);
  p:=p-(n2-n);
  writeln(a,' ',p);
end.

Your program writes 6 2
the answer is 11 2