import sys
sys.setrecursionlimit(200050)

def root(x):
    if P[x] < 0:
        return x
    else:
        P[x] = root(P[x])  # 経路圧縮
        return P[x]

def unite(x, y):
    x = root(x)
    y = root(y)
    if x == y:
        P[x] -= 1
        return
    if x > y: x,y = y,x
    P[x] = min(P[x], P[y]) - 1
    P[y] = x

def same(x, y):
    return root(x) == root(y)

def size(x):
    x = root(x)
    return -P[x]

N, M = map(int, input().split())
UV = [list(map(int, input().split())) for _ in range(M)]
P = [-1] * (N + 1)
cnt = 0
for u, v in reversed(UV):
    unite(u, v)
print(- P[1] - 1)