Merge Intervals

Merge overlapping intervals and print each surviving interval as start-end, one per line.

1-6
8-12
15-18

Sort by start. If the next interval starts at or before the current end, extend the end to the max of the two.

intervals = [(1, 3), (8, 10), (2, 6), (15, 18), (9, 12)]
merged = []
for start, end in sorted(intervals):
    if merged and start <= merged[-1][1]:
        merged[-1][1] = max(merged[-1][1], end)
    else:
        merged.append([start, end])
for start, end in merged:
    print(f"{start}-{end}")

Run your code to check it…