This program illustrates the use of the distance() function
to compute the number of positions between two iterators.
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Here are the values in a list of integers:
2 4 6 8 10 12 14 16 18 20
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p1 is pointing at the value 8 (the 4th value).
p2 is pointing at the value 18 (the 9th value).
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The number of values (or positions) "between" these two values
(inclusive) is 6 (= 9-4+1).
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