N = int(input())
link = [[] for _ in range(N+1)]
visited = [False] * (N+1)

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

def dfs(now) :
    visited[now] = True
    for nt in link[now] :
        if not visited[nt] :
            dfs(nt)

dfs(1)
print(visited.count(True)-1)