If ‘\( R \)’ is the least value of ‘\( a \)’ such that the function \( f(x) = x^{2} + ax + 1 \) is increasing on \( [1, 2] \) and ‘\( S \)’ is the Greatest value of ‘\( a \)’ such that the function \( f(x) = x^{2} + ax + 1 \) is decreasing on \( [1, 2] \), then the value of \( |R - S| \) is __________ .