from collections import deque 

n=int(input())
m=int(input())

graph=[[] for _ in range(n+1)]

for i in range(m):

    u,v=map(int,input().split())

    graph[u].append(v)
    graph[v].append(u)
 
vis = [False] * (n+1)

dq = deque()
dq.append(1)
vis[1] = True

while dq:
    cur = dq.popleft()

    for next in graph[cur]:
        if vis[next] == False:
            vis [next] = True
            dq.append(next)

print(vis.count(True)-1)