One, Two, Skip a Few

Create a function which calculates how many numbers are missing from an ordered number line. This number line starts at the first value of the list, and increases by 1 to the end of the number line, ending at the last value of the list.

how_many_missing([1, 2, 3, 8, 9]) ➞ 4

# The number line starts at 1 and ends at 9 (so the numbers 0 and 10 aren't missing from it).
# The numbers missing from this line are 4, 5, 6, and 7.
# 4 numbers are missing.

Examples

how_many_missing([1, 3]) ➞ 1

how_many_missing([7, 10, 11, 12]) ➞ 2

how_many_missing([1, 3, 5, 7, 9, 11]) ➞ 5

how_many_missing([5, 6, 7, 8]) ➞ 0

Notes

If the number line is complete, or the list is empty, return 0.