Complex sentence: The machine can only add the value of a column n + 1 to column n to produce the new value of n. Column N can only store a constant, column 1 displays (and possibly prints) the value of the calculation on the current iteration.
Split into simpler sentences: The machine can only add the value of a column "n" + 1 to column "n" to produce the new value of "n". Column "N" can only store a, column 1 displays (and possibly prints) the value of the calculation on the current iteration.