Let the set of all values of \( p \), for which \( f(x) = (p^2 - 6p + 8)(\sin^2 2x - \cos^2 2x) + 2(2 - p)x + 7 \) does not have any critical point, be the interval \( (a, b) \). Then \( 16ab \) is equal to ________.