#include <iostream>
#include <string>
#include <vector>
#include <set>
#include <stack>
#include <queue>
#include <map>
#include <algorithm>
#include <iomanip>
#include <math.h>
#include <string.h>
#include <cstdio>
#include <tuple>
#include <numeric>
#include <functional>
using namespace std; using ll = long long; using ld = long double;  using pll = pair<ll, ll>;
using vl = vector<ll>; using vll = vector<vl>; using vlll = vector<vll>; using vpll = vector<pll>;
using vs = vector<string>; using tll = tuple<ll, ll, ll>; using vtll = vector<tll>;
template<class T>bool chmax(T& a, const T& b) { if (a < b) { a = b; return 1; } return 0; }
template<class T>bool chmin(T& a, const T& b) { if (b < a) { a = b; return 1; } return 0; }
const ld PI = 3.1415926535897932;
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#define mp make_pair
#define mt make_tuple
#define pb push_back
#define co(i) cout << i << endl;
#define co2(i,j) cout << i << " " << j << endl;
#define co3(i) cout << i << " ";
#define po(i) cout << i.first << " " << i.second << endl;
void in() {}
void debug_out() { cerr << endl; }
template<typename Head, typename... Tail>
void debug_out(Head h, Tail... t) {
	cerr << " " << h;
	if (sizeof...(t) > 0) cerr << " :";
	debug_out(t...);
}
template <typename T>
ostream& operator<<(ostream& os, vector<T> vec) {
	for (size_t i = 0; i < vec.size(); i++)os << vec[i] << (i + 1 == vec.size() ? "" : " ");
	return os;
}
ll ctoi(char c) {
	switch (c) {
	case '0': return 0; case '1': return 1; case '2': return 2;
	case '3': return 3; case '4': return 4; case '5': return 5;
	case '6': return 6; case '7': return 7; case '8': return 8;
	case '9': return 9;  default: return 0;
	}
}
bool pairCompare(const pll firstElof, pll secondElof)
{
	return firstElof.second < secondElof.second;
}
//AtCoder以外ではコメントアウトを忘れずに！
#include <atcoder/all>
using namespace atcoder;
//**Snippetリスト**//
//rep, vin, all, iteLoop, bitSearch, bitList, nod, LIS, digitDP, treeDP, Rerooting//
//gcdlcm, isPrime, eratos, primeFactorize, Npow, combination, divisor, modinv, doubling//
//dfs, bfs, dijkstra, WarshallFloyd, BellmanFord, UnionFind, Kruskal, RMQ, LCA, FordFulkerson//
//**AC Libraryリスト**//
//pow_mod, inv_mod, floor_sum, convolution, convolution_ll, //
ll i, j, k, l; ll N, M, K, H, W, X, Y, Z, R, Q, T;
ll MOD = 1000000007, INF = 1LL << 60, ans = 0, z = 0, o = 1;
vl flag, D; vll path; vs S;
//***********//
class UnionFind {
public:
	vl rank, par, siz;
	UnionFind() {}
	UnionFind(ll N) {
		rank.resize(N, 0);
		par.resize(N, 0);
		siz.resize(N, 1);
		init(N);
	}
	void init(ll N) {
		for (ll i = 0; i < N; i++) {
			par[i] = i;
			rank[i] = 0;
		}
	}
	ll find(ll x) {
		if (par[x] == x) {
			return x;
		}
		else {
			return par[x] = find(par[x]);
		}
	}
	void unite(ll x, ll y) {
		x = find(x);
		y = find(y);
		if (x != y) {
			if (rank[x] < rank[y]) {
				par[x] = y;
				siz[y] += siz[x];
			}
			else {
				par[y] = x;
				siz[x] += siz[y];
				if (rank[x] == rank[y]) rank[x]++;
			}
		}
	}
	bool issame(ll x, ll y) {
		return find(x) == find(y);
	}
	ll size(ll x) {
		return siz[find(x)];
	}
};
int main() {
	cin >> N >> M;
	UnionFind UF(N);
	for (i = 0; i < M; i++) {
		ll a, b;
		cin >> a >> b;
		a--; b--;
		UF.unite(a, b);
	}
	flag.resize(N);
	ans = 0;
	for (i = 0; i < N; i++) {
		ll par = UF.find(i);
		//debug(par);
		if (!flag[par]) {
			ans++;
			flag[par] = 1;
		}
	}
	cout << ans - 1;
}


Measured memory_bytes: 6464