Influenza AH1N1#341
Any epidemic is dangerous, especially one as easily spread as influenza. As a result, governments of almost all countries develop special plans for such situations. One of the most effective measures during an epidemic is the isolation of the infected. Sometimes, entire cities must be isolated (what to do within an isolated city is decided by its local council). It is necessary to ensure that, if a city is isolated, communication between the remaining cities is preserved. If not, new roads must be built (preferably as few as possible).
The president of a country with a developed road network has asked your team to assess the situation. You are tasked with determining whether, if any city is isolated (that is, all roads leading to that city are blocked), communication between all other cities remains (in this country, initial communication between all cities exists). If there is at least one city whose isolation breaks communication, determine whether building just one road is sufficient so that the isolation of any city does not break communication. Since information about all roads in the country is strategic, the president specified cities by numbers instead of names.
The first line contains two integers separated by a space: (number of cities, 2 \le N \le 20000) and (number of roads, 1 \le M \le 200000). No two cities are connected by more than one road.
Each of the next lines contains two integers separated by a space, the numbers of the cities connected by each road.
If preserving connectivity when isolating any city is possible by building at most one new road, output YES; otherwise, output NO.
If it is necessary to build exactly one road to preserve connectivity when isolating any city, output YES 1.
Example 1