// ************************.- J A I  S H R E E  R A M -.*************************


/*
      ::: All is One ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

          Author   :  Udit "luctivud" Gupta

          linkedin :  https://www.linkedin.com/in/udit-gupta-1b7863135/
          github   :  https://github.com/luctivud

      :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: One is All :::
*/


#include <bits/stdc++.h>

#pragma GCC optimize "trapv"
#pragma GCC optimize ("Ofast")
// #pragma GCC target ("fpmath=sse,sse2")
#pragma GCC target ("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx")
#pragma GCC optimize ("-ffloat-store")


using namespace std;



typedef long long int lld;
typedef unsigned long long int llu;

#define         TESTCASES()    cin >> (T3X0); T353 = T3X0; while(T3X0--)
#define          input(V3C)    for(auto &V3C_I7 : (V3C)) cin >> (V3C_I7)
#define   mems(A77AY, V4LU)    memset((A77AY), (V4LU), sizeof((A77AY)))
#define    CH3K(I7, E4, S7)    (((S7)>0) ? (I7)<(E4) : (I7)>(E4))
#define   for4(I7,S4,E4,S7)    for(auto I7=(S4); CH3K(I7,E4,S7); (I7)+=(S7))
#define        forn(I7, E4)    for(lld I7=0ll; I7 < E4; (I7)+=1ll)
#define       forn1(I7, E4)    for(lld I7=1ll; I7 < E4+1; (I7)+=1ll)
#define        EACH(I7, A7)    for (auto& I7: A7)
#define              len(v)    ((int)((v).size()))
#define              all(x)    (x).begin(), (x).end()
#define             rall(x)    (x).rbegin(), (x).rend()
#define                  pb    push_back
#define         debhairu(x)    cout << #x << " = "; println(x);
#define         debspace(x)    cout << #x << " = "; print(x); cout << " ";
#define                  f1    first
#define                  s2    second
#define           PLEASE_AC    return



#define error(args...) { string _s = #args; replace(_s.begin(), _s.end(), ',', ' '); stringstream _ss(_s); istream_iterator<string> _it(_ss); huehue(_it, args); cout << "\n";}

void huehue(istream_iterator<string> it) {}
template<typename T, typename... Args>
void huehue(istream_iterator<string> it, T a, Args... args) {
	cout << *it << " = " << a << ", ";
	huehue(++it, args...);
}


template <class T> T inf() {
	return numeric_limits<T>::max();
}


void read() { return; }
void print() { return; }
void println() { cout << "\n"; return; }
template<class T> T read(T& x)   { cin >> x; return x; }
template<class T> void print(T a)   { cout << a; }
template<class T> void println(T a) { cout << a << "\n"; }

template<class T> void read(vector<T> &arr)   { EACH(i, arr) cin >> (i); }
template<class T> void print(vector<T> arr)   { EACH(i, arr) {cout << i << " ";} }
template<class T> void println(vector<T> arr) { EACH(i, arr) {cout << i << " ";} cout << "\n"; }

template<class T> void read(vector<vector<T>> &arr)   { EACH(i, arr) read(i); }
template<class T> void print(vector<vector<T>> arr)   { EACH(i, arr) println(i); }
template<class T> void println(vector<vector<T>> arr) { EACH(i, arr) println(i); }

template<typename T, typename... Args> void read(vector<T> &arr, Args &... args)   { read(arr); read(args...);}
template<typename T, typename... Args> void read(vector<vector<T>> &arr, Args &... args)   { read(arr); read(args...);}
template<typename T, typename... Args> void read(T &a, Args &... args) { cin >> (a); read(args...); }
template<typename T, typename... Args> void print(vector<T> &arr, Args &... args)   { print(arr); print(args...);}
template<typename T, typename... Args> void print(T a, Args... args) { cout << a << " "; print(args...); };
template<typename T, typename... Args> void println(vector<T> &arr, Args &... args)   { print(arr); println(args...);}
template<typename T, typename... Args> void println(T a, Args... args) { cout << a << " "; println(args...); };


const lld d4i[4] = { -1, 0, 1, 0}, d4j[4] = {0, 1, 0, -1};
const lld d8i[8] = { -1, -1, 0, 1, 1, 1, 0, -1}, d8j[8] = {0, 1, 1, 1, 0, -1, -1, -1};



const auto start_time = std::chrono::high_resolution_clock::now();

signed light() {

	ios_base::sync_with_stdio(false); cin.tie(0);
	cout.precision(10); cout << fixed;

#ifdef LUCTIVUD
	freopen("/home/luctivud/CPPractice/Zinput.txt", "r", stdin);
	freopen("/home/luctivud/CPPractice/Zoutput.txt", "w", stdout);
#endif

	return 0;
}


signed luctivud() {

#ifdef LUCTIVUD
	auto end_time = std::chrono::high_resolution_clock::now();
	std::chrono::duration<double> diff = end_time - start_time;
	cerr << "Finished in : " << diff.count() << "\n";
#endif

	return 0;
}



const lld MOD = lld(1e9) + 7ll;
const lld mod = MOD;


const long double EPS = 1e-6;

const lld INFll = inf<lld>();
const int INFi = inf<int>();


void add_undirected_edge(lld a, lld b, vector<lld> adj[]) {
	adj[a].pb(b);
	adj[b].pb(a);
	return;
}



/*
:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
*/


/*          D S U         */

void makeSet(lld n, vector<lld> &parent, vector<lld> &rank) {
	iota(all(parent), 0ll);
}

lld find_repr(lld x, vector<lld> &parent, vector<lld> &rank) {
	if (x != parent[x])
		parent[x] = find_repr(parent[x], parent, rank);
	return parent[x];
}

void Union(lld x, lld y, vector<lld> &parent, vector<lld> &rank) {
	x = find_repr(x, parent, rank);
	y = find_repr(y, parent, rank);

	if (x == y) return;

	if (rank[x] < rank[y]) {
		parent[x] = y;
	} else if (rank[x] > rank[y]) {
		parent[y] = x;
	} else {
		parent[y] = x;
		rank[x] = rank[x] + 1;
	}
	return;
}





void solveEachTest(lld _TestCase) {
	// cout << "Case#" << _TestCase << ": ";
	lld n, m; read(n, m);
	// create parent and rank at the place.
	vector<lld> parent(n, 0);
	vector<lld> rank(n, 0);
	makeSet(n, parent, rank);

	forn(i, m) {
		lld a, b; read(a, b);
		a--, b--;
		Union(a, b, parent, rank);
	}

	set<lld> components;
	forn(i, n) {
		components.insert(find_repr(i, parent, rank));
	}

	println(len(components) - 1);


	// cout << "\n";
	return;
}


signed main() {

	light(); lld T3X0 = 0, T353 = 1;

	// TESTCASES()
	solveEachTest(T353 - T3X0);

	PLEASE_AC luctivud();
}

// Random Thought :  null
/*
        My lungs will fill and then deflate
        They fill with fire, exhale desire
*/


Measured memory_bytes: 8540