Frequent Kitchen Duo Finder

A regional cooking competition keeps a running log of every dish prepared during the season. Each log entry records which head chef led the dish and which sous chef assisted them. Two people form a "seasoned duo" if they have worked together in that exact head-chef/sous-chef arrangement on at least 3 different dishes over the season. Given the full log, list every seasoned duo.

Input format

Line 1: a single integer n, the number of dish entries in the log.

Next n lines: two integers head_chef_id and sous_chef_id -- the ids of the head chef and sous chef for that dish. The same ordered pair of ids may repeat across multiple entries (once per dish they cooked together), and a chef's id may appear as a head chef in one entry and as a sous chef in another.

Output format

First print a single integer k, the number of seasoned duos. Then print k lines, each containing head_chef_id sous_chef_id for one seasoned duo, sorted ascending primarily by head_chef_id and secondarily by sous_chef_id. If no duo qualifies, print 0 and nothing further.

Constraints

• 1 <= n <= 10000
• 1 <= head_chef_id, sous_chef_id <= 100000
• head_chef_id and sous_chef_id may be equal within a single entry
• (head_chef_id, sous_chef_id) is an ordered pair: a duo with ids swapped is a different duo

8
1 1
1 1
1 1
1 2
1 2
2 1
2 1
2 2

1
1 1

7
2 3
2 3
2 3
2 3
1 5
1 5
1 5

2
1 5
2 3