What's Wrong?

var
   a: array [1..1000] of longint;
   i,j: integer;
   r: real;
begin
   i:=0;
   while not seekeof do begin
      inc(i);
      readln(a[i]);
   end;
   for j:=i downto 1 do begin
      r:=sqrt(a[j]);
      writeln(r:4:4);
   end;
end.

Type "longint" is too small for numbers from interval [0, 10^18]. You should use type "qword".
And there can be more than 1000 numbers in the input.