Two functions f₁ and f₂ are given as follows.

\(\begin{array}{l} {f_1}\left( {A,B,C,D} \right) = \sum \left( {{m_0},{m_1},{m_3},{m_4},{m_5},{m_7},{m_9},{m_{13}},{m_{14}},{m_{15}}} \right)\\ {f_2}\left( {A,B,C,D} \right) = \pi \left( {{M_0},{M_1},{M_6},{M_8},{M_{11}},{M_{12}}} \right) \end{array}\)

Then the number of essential prime implicants of the function f₃ = f₁f₂ is equal to, where f₃ is represented sum of products