Hungary says its immigration problem has reached unprecedented levels.More than 100,000 migrants have entered so far this year.A four-metre high fence is being built along its border with Serbia, in a desperate attempt to keep migrants away.The journey to the Serbia-Hungary border usually starts after migrants arrive in Greece, often from Africa or the Middle East.The migrants then move into Albania, Serbia and into Slovenia and Hungary.The construction of the fence in Hungary makes this last route nearly impossible to pass.Al Jazeera's Laurence Lee sent this report from the border.Fight now or never.