Number of Covered Integers

Write a function that returns the total number of integers covered from a list of intervals (inclusive). In other words, return the number of different integers in the lists.

Examples

covered_integers([[80, 81], [1, 2], [9, 11]]) ➞ 7
# Seven integers are covered: 1, 2, 9, 10, 11, 80, 81

covered_integers([[3, 6], [4, 6], [5, 6]]) ➞ 4

covered_integers([[1, 2], [1, 2]]) ➞ 2

Notes

• Intervals may overlap, be subintervals of each other, or be identical.
• For each interval [l, u], l and u will be integers, and l <= u will always be true.