//
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//
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//       ?77WMMMYB""!                                                                        _7"WWMMMY"7=     

// need
#include <iostream>
#include <algorithm>
// data structure
#include <bitset>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <cstring>
#include <utility>
#include <vector>
#include <complex>
//#include <deque>
#include <valarray>
#include <unordered_map>
#include <unordered_set>
#include <array>
// etc
#include <cassert>
#include <cmath>
#include <functional>
#include <iomanip>
#include <chrono>
#include <random>
#include <numeric>
#include <fstream>

//std::ifstream ifs("b.in");
//auto& scan_in = ifs;
auto& scan_in = std::cin;
auto& scan_out = std::cout;

// input
#define INIT std::ios::sync_with_stdio(false);std::cin.tie(0);
#define VAR(type, ...)type __VA_ARGS__;MACRO_VAR_Scan(__VA_ARGS__);
template<typename T> void MACRO_VAR_Scan(T & t) { scan_in >> t; }
template<typename First, typename...Rest>void MACRO_VAR_Scan(First & first, Rest & ...rest) { scan_in >> first; MACRO_VAR_Scan(rest...); }
#define VEC_ROW(type, n, ...)std::vector<type> __VA_ARGS__;MACRO_VEC_ROW_Init(n, __VA_ARGS__); for(int w_=0; w_<n; ++w_){MACRO_VEC_ROW_Scan(w_, __VA_ARGS__);}
template<typename T> void MACRO_VEC_ROW_Init(int n, T & t) { t.resize(n); }
template<typename First, typename...Rest>void MACRO_VEC_ROW_Init(int n, First & first, Rest & ...rest) { first.resize(n); MACRO_VEC_ROW_Init(n, rest...); }
template<typename T> void MACRO_VEC_ROW_Scan(int p, T & t) { scan_in >> t[p]; }
template<typename First, typename...Rest>void MACRO_VEC_ROW_Scan(int p, First & first, Rest & ...rest) { scan_in >> first[p]; MACRO_VEC_ROW_Scan(p, rest...); }
#define VEC(type, c, n) std::vector<type> c(n);for(auto& i:c)scan_in>>i;
#define MAT(type, c, m, n) std::vector<std::vector<type>> c(m, std::vector<type>(n));for(auto& R:c)for(auto& w:R)scan_in>>w;
// output
template<typename T>void MACRO_OUT(const T t) { scan_out << t; }
template<typename First, typename...Rest>void MACRO_OUT(const First first, const Rest...rest) { scan_out << first << ' '; MACRO_OUT(rest...); }
#define OUT(...) MACRO_OUT(__VA_ARGS__);
#define FOUT(n, dist) scan_out<<std::fixed<<std::setprecision(n)<<(dist);
#define SOUT(n, c, dist) scan_out<<std::setw(n)<<std::setfill(c)<<(dist);
#define VOUT(v) for(size_t i = 0; i < v.size(); ++i) {OUT(v[i]);if(i+1<v.size()){SP}}
#define EOUT(...) do{ OUT(__VA_ARGS__)BR; exit(0); }while(0);
#define SP scan_out<<' ';
#define TAB scan_out<<'\t';
#define BR scan_out<<'\n';
#define SPBR(w, n) scan_out<<(w + 1 == n ? '\n' : ' ');
#define ENDL scan_out<<std::endl;
#define FLUSH scan_out<<std::flush;
#define SHOW(dist) {std::cerr << #dist << "\t: " << (dist) << '\n';}
// utility
#define ALL(a) (a).begin(),(a).end()
#define FOR(w, a, n) for(int w=(a);w<(n);++w)
#define REP(w, n) FOR(w, 0, n)
#define RFOR(w, a, n) for(int w=(n)-1;w>=(a);--w)
#define RREP(w, n) RFOR(w, 0, n)
template<class S, class T, class U> bool IN(S a, T x, U b) { return a <= x && x < b; }
template<class T> inline T CHMAX(T & a, const T b) { return a = (a < b) ? b : a; }
template<class T> inline T CHMIN(T & a, const T b) { return a = (a > b) ? b : a; }
// test
template<class T> using V = std::vector<T>;
template<class T> using VV = V<V<T>>;

template<typename S, typename T>
std::ostream& operator<<(std::ostream & os, const std::pair<S, T> & p) {
	os << '(' << p.first << ',' << p.second << ')';
	return os;
}
template<typename T>
std::ostream& operator<<(std::ostream & os, const std::vector<T> & v) {
	os << '{';
	for (size_t i = 0; i < v.size(); ++i) os << v[i] << ((i + 1 < v.size()) ? ',' : '}');
	return os;
}
template<typename T>
std::ostream & operator<<(std::ostream & os, const std::set<T> & v) {
	os << '{';
	for (auto it = v.cbegin(); it != v.cend();) {
		os << *it;
		os << (++it != v.cend()) ? '}' : ',';
	}
	return os;
}
template<typename S, typename T>
std::ostream& operator<<(std::ostream & os, const std::map<S, T> & m) {
	os << '{';
	for (auto it = m.cbegin(); it != m.cend();) { os << it->first << ':' << it->second; ++it; os << (it != m.cend() ? ',' : '}'); }
	return os;
}
template<typename T>
std::ostream& operator<<(std::ostream & os, std::queue<T> q) {
	os << '<';
	while (!q.empty()) { os << q.front(); q.pop(); os << (q.empty() ? '<' : ','); }
	return os;
}
template<typename T>
std::ostream& operator<<(std::ostream & os, std::stack<T> q) {
	os << '>';
	while (!q.empty()) { os << q.top(); q.pop(); os << (q.empty() ? ']' : ','); }
	return os;
}
namespace std {
	template<typename S, typename T> class numeric_limits<pair<S, T>> {
	public:
		static constexpr pair<S, T> max() noexcept { return { numeric_limits<S>::max(), numeric_limits<T>::max() }; }
		static constexpr pair<S, T> lowest() noexcept { return { numeric_limits<S>::lowest(), numeric_limits<T>::lowest() }; }
	};
}

// type/const
using i64 = std::int_fast64_t;
using u64 = std::uint_fast64_t;
using ll = long long;
using ull = unsigned long long;
using ld = long double;
using PAIR = std::pair<i64, i64>;
using PAIRLL = std::pair<ll, ll>;
constexpr int INFINT = (1 << 30) - 1;                    // 1.07x10^ 9
constexpr int INFINT_LIM = (1LL << 31) - 1;              // 2.15x10^ 9
constexpr ll INFLL = 1LL << 60;                          // 1.15x10^18
constexpr ll INFLL_LIM = (1LL << 62) - 1 + (1LL << 62);  // 9.22x10^18
constexpr double EPS = 1e-6;
constexpr i64 MOD = 998244353;
constexpr double PI = 3.141592653589793238462643383279;
template<class T, size_t N> void FILL(T(&a)[N], const T & val) { for (auto& x : a) x = val; }
template<class ARY, size_t N, size_t M, class T> void FILL(ARY(&a)[N][M], const T & val) { for (auto& b : a) FILL(b, val); }
template<class T> void FILL(std::vector<T> & a, const T & val) { for (auto& x : a) x = val; }
template<class ARY, class T> void FILL(std::vector<std::vector<ARY>> & a, const T & val) { for (auto& b : a) FILL(b, val); }
// ------------>8---------

class UnionFind {
private:
	std::vector<int> par;
	std::vector<int> siz;

public:
	UnionFind(int sz_) : par(sz_), siz(sz_, 1) {
		for (int i = 0; i < sz_; ++i) par[i] = i;
	}
	void init(int sz_) {
		par.resize(sz_);
		siz.resize(sz_, 1);
		for (int i = 0; i < sz_; ++i) par[i] = i;
	}
	int find(int x) {
		while (par[x] != x) x = par[x] = par[par[x]];
		return x;
	}
	void unite(int x, int y) {
		x = find(x);
		y = find(y);
		if (x == y) return;
		if (siz[x] < siz[y]) std::swap(x, y);
		siz[x] += siz[y];
		par[y] = x;
	}
	bool same(int x, int y) {
		return find(x) == find(y);
	}
	int size(int x) {
		return siz[find(x)];
	}
};

signed main() {
	INIT;

	VAR(int, n, m);

	UnionFind uf(n);
	REP(_, m) {
		VAR(int, a, b); --a; --b;
		uf.unite(a, b);
	}

	std::set<int> set;
	REP(i, n) set.insert(uf.find(i));

	OUT(set.size() - 1)BR;

	return 0;
}

Measured memory_bytes: 7948