How many B's are there which are immediately preceded by A but not immediately followed by A.B Z A N A A B Z A B A Z B A P Z A B A Z A
P(z)=z4+az3+bz2+cz+d is a polynomial where a, b, c and d are real numbers. Find a+b+c+d if two zeros of polynomial P are the following complex numbers: 2−i and 1−i.