import sys
n=int(sys.stdin.readline().rstrip())
link=int(sys.stdin.readline().rstrip())
linked=[[] for _ in range(n+1)]
visited=[False]*(n+1)
for _ in range(link):
    x,y=map(int,sys.stdin.readline().rstrip().split(" "))
    linked[x].append(y)
    linked[y].append(x)
# print(linked)
stack=[1]

while len(stack)!=0:
    checking=stack.pop()
    if visited[checking]==True:
        continue
    visited[checking]=True
    if linked[checking]!=0:
        stackinput=sorted(linked[checking])
        while len(stackinput)!=0:
            stack.append(stackinput.pop())
    else:
        pass

print(visited.count(True)-1)