**$|x-y| = $**difference between the numbers

**$x$**and

**$y$**. For example,

**$|5-2| = 3, |3-9| = 6$**. Let

**$a_1, a_2, a_3, \cdots , a_n$**be a sequence of numbers such that each term in the sequence is larger than the previous term.

Let

**$S = |a_1 - a_2| + |a_2 - a_3|+ \cdots + |a_{n-1} - a_n|$**. What is the minimum number of numbers that you need to know from the sequence in order to find

**$S$**?