Python3: Sum of Intervals

Kata

https://www.codewars.com/kata/sum-of-intervals/python

Write a function called sumIntervals/sum_intervals() that accepts an array of intervals, and returns the sum of all the interval lengths. Overlapping intervals should only be counted once.

Intervals
Intervals are represented by a pair of integers in the form of an array. The first value of the interval will always be less than the second value. Interval example: [1, 5] is an interval from 1 to 5. The length of this interval is 4.

Overlapping Intervals
List containing overlapping intervals:

[
   [1,4],
   [7, 10],
   [3, 5]
]

The sum of the lengths of these intervals is 7. Since [1, 4] and [3, 5] overlap, we can treat the interval as [1, 5], which has a length of 4.

Examples:

sumIntervals( [
   [1,2],
   [6, 10],
   [11, 15]
] ); // => 9

sumIntervals( [
   [1,4],
   [7, 10],
   [3, 5]
] ); // => 7

sumIntervals( [
   [1,5],
   [10, 20],
   [1, 6],
   [16, 19],
   [5, 11]
] ); // => 19

My Solution

def sum_of_intervals(intervals):
    numbers = []
    for interval in intervals:
        for i in range(interval[0],interval[1]):
            numbers.append(i)
    return len(set(numbers))