Dear Sir, in lecture no. 3 of electrostatics II, you gave two examples for dipole calculation. In the second example, there were four charges placed in a line like -ve -ve +ve +ve, you said that one -ve can be paired with only one +ve and not two, but I am not able to comprehend it, in just the example before it, which had one -ve and two +ve charges at 90 degree, you paired the -ve charge with both the +ve charges and calculated the dipole, then why not pair all the -ve with all the +ve in the second example?