백준 2606번 - 바이러스

vertexList = [i for i in range(int(input()))]
edgeList = []
for _ in range(int(input())):
  a,b=map(int,input().split())
  edgeList.append((a,b))
  edgeList.append((b,a))

adjacencyList = [[] for vertex in vertexList]

for edge in edgeList:
  adjacencyList[edge[0]-1].append(edge[1])

stack = [1]
visitedVertex = []

while stack:
  current = stack.pop()
  if current not in visitedVertex:
    visitedVertex.append(current)
    stack += adjacencyList[current-1]
    
print(len(visitedVertex)-1)