origin	destination	population
3	4	1.0
3	1	0.0
3	2	0.0
4	3	0.0
4	1	0.0
4	2	0.0
1	3	0.0
1	4	0.0
1	2	0.0
2	3	0.0
2	4	0.0
2	1	0.0