Concat is so concat

                var
                 s : ansistring;
                 i, n : longint;
                begin
                    for i := 0 to 49999 do
                        s := Concat( s, 'ab' );
                    for i := 0 to 49999 do
                        s := Concat( s, 'ba' );
                    s := Concat( s, 'c' );

                    n := length(s);
                    for i := n - 1 downto 1 do if (s[i] = s[i + 1]) then delete(s, i, 2);
                    writeln(s);
                end.