set NODES := 1 2 3 4 5 6;

set ARCS := (1,2) (1,4) (2,3) (2,4) (3,4) (4,5) (3,6) (5,6);

param: c :=
1 2 5
1 4 7
2 3 -2
2 4 1
3 4 2
4 5 2
3 6 6
5 6 1;