�点数和边数． 
接下来m行每行两个数a, b表示有一条无向边<a, b>． 
</div></div></div></div></div><div class="row"><div class="column"><h4 class="ui top attached block header">输出格式</h4><div class="ui bottom attached segment font-content"><div style="position: relative; overflow: hidden;"><div class=content>一个整数表示方案总数． 
</div></div></div></div></div><div class="row"><div class="column"><h4 class="ui top attached block header">样例</h4><div class="ui bott