Glider

A plane is flying at a constant height of $$$h$$$ meters above the ground surface. Let's consider that it is flying from the point $$$(-10^9, h)$$$ to the point $$$(10^9, h)$$$ parallel with $$$Ox$$$ axis.

A glider is inside the plane, ready to start his flight at any moment (for the sake of simplicity let's consider that he may start only when the plane's coordinates are integers). After jumping from the plane, he will fly in the same direction as the plane, parallel to $$$Ox$$$ axis, covering a unit of distance every second. Naturally, he will also descend; thus his second coordinate will decrease by one unit every second.

There are ascending air flows on certain segments, each such segment is characterized by two numbers $$$x_1$$$ and $$$x_2$$$ ($$$x_1 < x_2$$$) representing its endpoints. No two segments share any common points. When the glider is inside one of such segments, he doesn't descend, so his second coordinate stays the same each second. The glider still flies along $$$Ox$$$ axis, covering one unit of distance every second.

If the glider jumps out at $$$1$$$, he will stop at $$$10$$$. Otherwise, if he jumps out at $$$2$$$, he will stop at $$$12$$$.

Determine the maximum distance along $$$Ox$$$ axis from the point where the glider's flight starts to the point where his flight ends if the glider can choose any integer coordinate to jump from the plane and start his flight. After touching the ground the glider stops altogether, so he cannot glide through an ascending airflow segment if his second coordinate is $$$0$$$.