1 2
2 3
3 1
1 10
in this test , i do not know why ?
what about the others vertex ? 4,5,6,7,8,9 ??

"Imagine graph drawn on a wall. Select any vertex and erase exactly two edges incidental to this vertex. The question of problem is: "Is it possible to erase all edges of graph doing in this way?""

think again:
1 2
2 3
3 1
1 10

you will understand....