\problemname{Meticulous smoothing}
\illustration{.3}{pic}{\href{https://commons.wikimedia.org/wiki/File:Yew-wood.jpg}{CC-BY-SA 4.0, Tim van de Staak via Wikimedia Commons}}%
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The arts and crafts teacher is looking at the beautiful plank you crafted in the woodshop, and gaze at you with a stern look. ``This is not smooth enough! Use more sandpaper!''

Your plank is $n$ cm long, and the arts and crafts teacher has measured the width of your plank on $k$ different locations to prove his point. He demands that the thickness should differ by no more than $1$ micrometer between any two consecutive measured location. If the sandpaper will shave off $1$ micrometer of wood each time you use it at a particular location, how many times do you need to use the sandpaper?

\section*{Input}
The first line of input contains a single integer $1 \leq n \leq 10^6$, the length of your plank. On the second line of input follows $n$ space-separated integers $k_1, k_2, \ldots, k_n$, the thickness of your plank ($1 \leq k_i \leq 10^6$ for every $i$).

\section*{Output}
Output a single integer, the minimum number of times you need to use the sandpaper (assuming that the sandpaper only touch one location at the same time).
