import sys
from collections import deque

N = int(sys.stdin.readline().strip())
M = int(sys.stdin.readline().strip())
graph = dict()

for i in range(1, N+1):
    graph[i] = []

for _ in range(M):
    key, value = map(int, sys.stdin.readline().split())
    graph[key].append(value)
    if not key in graph[value]:
        graph[value].append(key)

def bfs(graph):
    visit = list()
    queue = deque()
    queue.append(1)

    while queue:
        node = queue.popleft()
        if node not in visit:
            visit.append(node)
            queue.extend(graph[node])
    if visit:
        return len(visit)-1
    else:
        return 0

print(bfs(graph))