### BUGGY CODE ###

# Template 1.0
import sys, re
from collections import deque, defaultdict, Counter, OrderedDict
from math import ceil, sqrt, hypot, factorial, pi, sin, cos, radians
from heapq import heappush, heappop, heapify, nlargest, nsmallest
def STR(): return list(input())
def INT(): return int(input())
def MAP(): return map(int, input().split())
def LIST(): return list(map(int, input().split()))
def list2d(a, b, c): return [[c] * b for i in range(a)]
def sortListWithIndex(listOfTuples, idx):   return (sorted(listOfTuples, key=lambda x: x[idx]))
def sortDictWithVal(passedDic):
    temp = sorted(passedDic.items(), key=lambda kv: (kv[1], kv[0]))
    toret = {}
    for tup in temp:
        toret[tup[0]] = tup[1]
    return toret
def sortDictWithKey(passedDic):
    return dict(OrderedDict(sorted(passedDic.items())))
sys.setrecursionlimit(10 ** 9)
INF = float('inf')
mod = 10 ** 9 + 7


class Graph:
    # init function to declare class variables
    def __init__(self, V):
        self.V = V
        self.adj = [[] for i in range(V)]

    def DFSUtil(self, temp, v, visited):

        # Mark the current vertex as visited
        visited[v] = True

        # Store the vertex to list
        temp.append(v)

        # Repeat for all vertices adjacent
        # to this vertex v
        for i in self.adj[v]:
            if visited[i] == False:
                # Update the list
                temp = self.DFSUtil(temp, i, visited)
        return temp

        # method to add an undirected edge

    def addEdge(self, v, w):
        self.adj[v].append(w)
        self.adj[w].append(v)

        # Method to retrieve connected components

    # in an undirected graph
    def connectedComponents(self):
        visited = []
        cc = []
        for i in range(self.V):
            visited.append(False)
        for v in range(self.V):
            if visited[v] == False:
                temp = []
                cc.append(self.DFSUtil(temp, v, visited))
        return cc

        # Driver Code


if __name__ == "__main__":
    # Create a graph given in the above diagram
    # 5 vertices numbered from 0 to 4
    # g = Graph(5);
    # g.addEdge(1, 0)
    # g.addEdge(2, 3)
    # g.addEdge(3, 4)



    n,m = MAP()

    g = Graph(n)

    for _ in range(m):
        u,v = MAP()
        g.addEdge(u-1,v-1)
        g.addEdge(v-1,u-1)
    print(len(g.connectedComponents()) - 1)

### FIXED CODE ###

# Template 1.0
import sys, re
from collections import deque, defaultdict, Counter, OrderedDict
from math import ceil, sqrt, hypot, factorial, pi, sin, cos, radians
from heapq import heappush, heappop, heapify, nlargest, nsmallest
def STR(): return list(input())
def INT(): return int(input())
def MAP(): return map(int, input().split())
def LIST(): return list(map(int, input().split()))
def list2d(a, b, c): return [[c] * b for i in range(a)]
def sortListWithIndex(listOfTuples, idx):   return (sorted(listOfTuples, key=lambda x: x[idx]))
def sortDictWithVal(passedDic):
    temp = sorted(passedDic.items(), key=lambda kv: (kv[1], kv[0]))
    toret = {}
    for tup in temp:
        toret[tup[0]] = tup[1]
    return toret
def sortDictWithKey(passedDic):
    return dict(OrderedDict(sorted(passedDic.items())))
sys.setrecursionlimit(10 ** 9)
INF = float('inf')
mod = 10 ** 9 + 7

n,m = MAP()

graph = defaultdict(list)

for _ in range(m):
    u,v = MAP()
    graph[u].append(v)
    graph[v].append(u)

comp = 0
vis = [False]*(n+1)
for i in range(1,n+1):
    if(vis[i]==False):
        comp+=1
        stack = [i]
        while(stack):
            node = stack.pop()
            vis[node] = True
            for adj in graph[node]:
                if(vis[adj]==False):
                    stack.append(adj)



print(comp - 1)