CF840B Leha and another game about graph

Leha plays a computer game, where is on each level is given a connected graph with $ n $ vertices and $ m $ edges. Graph can contain multiple edges, but can not contain self loops. Each vertex has an integer $ d_{i} $ , which can be equal to $ 0 $ , $ 1 $ or $ -1 $ . To pass the level, he needs to find a «good» subset of edges of the graph or say, that it doesn't exist. Subset is called «good», if by by leaving only edges from this subset in the original graph, we obtain the following: for every vertex i, $ d_{i} $ = - 1 or it's degree modulo 2 is equal to $ d_{i} $ . Leha wants to pass the game as soon as possible and ask you to help him. In case of multiple correct answers, print any of them.

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In the first sample we have single vertex without edges. It's degree is 0 and we can not get 1.