Application Of Derivatives

If 'R' is the least value of 'a' such that the function f(x) = x² + ax + 1 is increasing on [1, 2] and 'S' is the greatest value of 'a' such that the function f(x) = x² + ax + 1 is decreasing on [1, 2], then
the value of |R \(-\) S| is ___________.