DIMACS graph output:
c created by igraph
p max 6 8
n 1 s
n 6 t
a 1 2 5
a 1 3 2
a 2 3 2
a 2 4 3
a 3 5 4
a 4 5 1
a 4 6 2
a 5 6 5

DIMACS graph output for null graph:
c created by igraph
p max 0 0
n 1 s
n 6 t