Input:
6
1 6
2 6
3 6
2 5
4 5
-1 -1
2 3
1 2
-1 -1

Expected output:

Source nodes: 1 2 3 4

Similar node pairs: (2, 3) (1, 2)

Collapsed graph:

4: 3 4 5 6
3 6
3 5
4 5