1/3-2/15+3/35-4/63+5/99-6/143
=1/3 - (1/3 - 1/5) + 3/2 (1/5 - 1/7) - 2 (1/7 - 1/9) + 5/2 (1/9 - 1/11) - 3 (1/11 - 1/13)