minimize phone_a+phone_b+phone_c+phone_d+phone_e+phone_f subject to phone_a+phone_c+phone_f>1 phone_a+phone_d>1 phone_a+phone_b+phone_e>1 phone_b+phone_c+phone_d+phone_f>1 phone_b+phone_d+phone_e>1 phone_c+phone_e>1 phone_a>0 phone_b>0 phone_c>0 phone_d>0 phone_e>0 phone_f>0 binary phone_a phone_b phone_c phone_d phone_e phone_f end