Complex sentence: The key point is that these FFTs are not of the same length "n": such a linear convolution can be computed exactly from FFTs only by zero-padding it to a length greater than or equal to 2 "n"--1.
Split into simpler sentences: The key point is that these FFTs are not of the same length "n". In particular, such a linear convolution can be computed exactly from FFTs only by zero-padding it to a length greater than or equal to 2 "n"--1.