Longest Clean Video Clip

A video file has a total duration of D seconds (from 0 to D). There are N intervals [start, end) where the video data is corrupted. Find the length of the longest 'clean' (non-corrupted) continuous segment in the video. Input Format: The first line contains two integers: D and N. The next N lines each contain the start and end of a corrupted interval. Output Format: Print the length of the longest clean segment. Example: Input: 100 2 10 20 50 80 Output: 30 Explanation: Clean segments are [0,10) (len 10), [20,50) (len 30), and [80,100) (len 20). The longest is 30.