Codility Web Programmer interview questions

Each record in this table represents a contiguous segment of a line, from l to r inclusive. Its length equals r − l. Consider the parts of a line covered by the segments. Write an SQL query that returns the total length of all the parts of the line covered by the segments specified in the table segments. Please note that any parts of the line that are covered by several overlapping segments should be counted only once. For example, given: