|1/101-1/102|+|1/102-1/103|+|1/103+1/104|+|1/104-1/105|

=1/101-1/102+1/102-1/103+1/103-1/104+1/104-1/105
=1/101-1/105
=（105-101）/（101*105）
=4/10605