Dual-Lamps Street

A straight street is illuminated by N lamps. Each lamp $i$ covers the interval $[S_i, E_i]$. Maintenance crews want to know which parts of the street are 'redundantly lit' by exactly two lamps. Calculate the total length of the street that is covered by exactly two overlapping lamp ranges. Input Format: - An integer N (number of lamps). - N lines, each containing two integers S and E. Output Format: - A single integer representing the total length covered by exactly 2 lamps. Example: Input: 3 1 10 5 15 8 20 Output: 8 Explanation: Range 1-5 (1 lamp), 5-8 (2 lamps), 8-10 (3 lamps), 10-15 (2 lamps), 15-20 (1 lamp). The exactly-two ranges are [5,8] and [10,15], length 3 + 5 = 8.