I found a proof of this that states, "the hypothesis of this test guarantee that the subsequence \{s_{2n}\} is an increasing sequence that is bounded above, while the subsequence \{s_{2n-1}\} is a decreasing sequence that is bounded below."

Then, s_{2n}=(a_{1}-a_{2})+(a_{3}-a_{4})+...+(a_{2n-1}-a_{2n})

I don't understand why the inequality holds in the second line and leads to the 3rd. Why is it guaranteed to be increasing?