#pragma GCC target("avx2")
#pragma GCC optimize("03")
#pragma GCC optimize("unroll-loops")
#include <bits/stdc++.h>
using namespace std; using ld = long double; using ll = long long;
using ull = unsigned long long;
#define endl "\n"
#define FOR(i,a,b) for(int i=(a);i<=(b);i++)
#define rep(i,n) for(int i=0;i<(n);i++)
#define PII pair<int, int>
#define PLL pair<ll, ll>
#define ALL(x) (x).begin(), (x).end()
#define QUERY int num_query;cin>>num_query; while(num_query--)
int dx[] = {1, 0, -1, 0}; int dy[] = {0, 1, 0, -1};
constexpr int INF=1<<30;
constexpr ll LINF=1LL<<60; 
constexpr ll mod=1e9+7;
void flush() {cout<<flush;}
template<class T>vector<T> vec(int len, T elem) { return vector<T>(len, elem); } // auto dp = vec(52, vec(103, vec(103, INF)));
template<class T>inline bool chmax(T &a, const T &b) { if (a<b) { a = b; return 1; } return 0; }
template<class T>inline bool chmin(T &a, const T &b) { if (b<a) { a = b; return 1; } return 0; }
template<class T>inline int popcount(T a) {return __builtin_popcountll(a);}
template<class T>inline T emod(T a, T p) { return (a%p + p) % p;}
template <typename T1, typename T2> ostream &operator<<(ostream &os, const pair<T1, T2> &pa) { os << '(' << pa.first << ',' << pa.second << ')'; return os; }
template <typename TK, typename TV> ostream &operator<<(ostream &os, const map<TK, TV> &mp) { os << '{'; for (auto v : mp) os << v.first << "=>" << v.second << ','; os << '}'; return os; }
template <typename TK, typename TV> ostream &operator<<(ostream &os, const unordered_map<TK, TV> &mp) { os << '{'; for (auto v : mp) os << v.first << "=>" << v.second << ','; os << '}'; return os; }
template <typename T> istream &operator>>(istream &is, vector<T> &vec) { for (auto &v : vec) is >> v; return is; }
template <typename T> ostream &operator<<(ostream &os, const vector<T> &vec) {os << '\n'; for (auto v : vec) os << v << ' '; os << '\n'; return os;}
template <typename T> ostream &operator<<(ostream &os, const deque<T> &vec) { os << "deq["; for (auto v : vec) os << v << ','; os << ']'; return os; }
template <typename T> ostream &operator<<(ostream &os, const set<T> &vec) { os << '{'; for (auto v : vec) os << v << ','; os << '}'; return os; }
template <typename T> ostream &operator<<(ostream &os, const unordered_set<T> &vec) { os << '{'; for (auto v : vec) os << v << ','; os << '}'; return os; }
template <typename T> ostream &operator<<(ostream &os, const multiset<T> &vec) { os << '{'; for (auto v : vec) os << v << ','; os << '}'; return os; }
template <typename T> ostream &operator<<(ostream &os, const unordered_multiset<T> &vec) { os << '{'; for (auto v : vec) os << v << ','; os << '}'; return os; }
struct MyIO {
    MyIO(){
        cin.tie(0); 
        ios::sync_with_stdio(false); 
        cout << fixed << setprecision(15);
    }
} IO_OI;
#ifdef LOCAL
void debug_out() { cerr << endl; }
template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
    cerr << " " << H;
    debug_out(T...);
}
#define debug(...) \
    cerr << __LINE__ << " [" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#define dump(x) cerr << __LINE__ << " " << #x << " = " << (x) << endl
#else
#define debug(...) (void(0))
#define dump(x) (void(0))
#endif
//-------------------
class UnionFind {
private:
	unsigned size_; std::vector<int> par, rank; //par = root? -size: par 
public:
	UnionFind() : size_(0), par(std::vector<int>()), rank(std::vector<int>()) {};
	UnionFind(unsigned size__) : size_(size__) {
		par.resize(size_+1); rank.resize(size_+1);
		for (unsigned i = 0; i <= size_; i++) par[i] = -1, rank[i] = 0;
	}
	//unsigned size() { return size_; }
	unsigned root(unsigned x) {return par[x] < 0 ? x : par[x] = root(par[x]); }
	bool same(unsigned x, unsigned y) { return root(x) == root(y); }
	void unite(unsigned x, unsigned y) {
		x = root(x), y = root(y);
		if (x == y) return;
		if (rank[x] < rank[y])par[y] += par[x], par[x] = y;
		else if (rank[x] == rank[y])par[x] += par[y], par[y] = x, rank[x]++;
		else par[x] += par[y], par[y] = x;
	}
	int size(int x){ return -par[root(x)];} 
	bool operator==(const UnionFind &u) { return par == u.par; }
	bool operator!=(const UnionFind &u) { return par != u.par; }
};
int main() {
    int n,m;cin>>n>>m;
    UnionFind uf(n);
    rep(i,m) {
        int a,b;cin>>a>>b;
        a--;b--;
        uf.unite(a,b);
    }

    unordered_map<int,int> mp;
    rep(i,n) {
        mp[uf.root(i)]++;
    }
    int ans = mp.size() - 1;
    cout << ans << endl;
}

Measured memory_bytes: 7348