you give n pair integer a_1 b_1 a_2 b_2 ldot a_n b_n all integer pair distinct range 1 2 cdot n inclusivelet sequence integer x_1 x_2 ldot x_2k good x_1 x_2 x_3 ldot x_2k2 x_2k1 x_2k x_1 x_2 x_3 ldot x_2k2 x_2k1 x_2k you need choose subset distinct index i_1 i_2 ldot i_t order way write number pair single sequence sequence a_i_1 b_i_1 a_i_2 b_i_2 ldot a_i_t b_i_t sequence goodwhat large subset index choose you need construct correspond index sequence i_1 i_2 ldot i_t