In a digital combinatorial circuit with 4 inputs (A, B, C, D) it is required to obtain an output of logical 1 only for the input combination (A = 1, B = C = D = 0). It is known that the following combinations of input are forbidden:

ABCD = 1010, 1011, 1100, 1101, 1110, 1111.

The minimum number of two input NAND gates required to implement the logic expression of the output are Assume that complements of inputs are not available.