2 + 4 + 6 + 8 + 10 +. + 96 + 98 + 100 is calculated by drag type

2 + 4 + 6 + 8 + 10 +. + 96 + 98 + 100 is calculated by drag type

(2+100)+(4+98)+(6+96)+················(50+52)
=102x50/2
=102x25
=2550