#include <bits/stdc++.h>
 #include <atcoder/all>

//tabaicho  see https://boostjp.github.io/tips/multiprec-int.html   
// #include <boost/multiprecision/cpp_int.hpp>
// using namespace boost::multiprecision;

// cpp_int
// int128_t
// int256_t
// int512_t
// int1024_t

// uint128_t
// uint256_t
// uint512_t
// uint1024_t



 #define int long long
 #define inf  1000000007
// #define inf  998244353

 #define pa pair<int,int>
 #define ll long long
 #define PI 3.14159265358979323846
 #define  mp make_pair
 #define  pb push_back
 #define EPS (1e-8)
 
      using namespace std;
                                          
 int dx[8]={0,1,0,-1,1,1,-1,-1};
 int dy[8]={1,0,-1,0,-1,1,1,-1};
                                            
class pa3{
	public:
	int x;
	int y,z;
	pa3(int x=0,int y=0,int z=0):x(x),y(y),z(z) {}
	bool operator < (const pa3 &p) const{
		if(x!=p.x) return x<p.x;
		if(y!=p.y) return y<p.y;
		 return z<p.z;
		//return x != p.x ? x<p.x: y<p.y;
	}
	bool operator > (const pa3 &p) const{
		if(x!=p.x) return x>p.x;
		if(y!=p.y) return y>p.y;
		 return z>p.z;
		//return x != p.x ? x<p.x: y<p.y;
	}
	bool operator == (const pa3 &p) const{
		return x==p.x && y==p.y && z==p.z;
	}
		bool operator != (const pa3 &p) const{
			return !( x==p.x && y==p.y && z==p.z);
	}

};

class pa4{
	public:
	int x;
	int y,z,w;
	pa4(int x=0,int y=0,int z=0,int w=0):x(x),y(y),z(z),w(w) {}
	bool operator < (const pa4 &p) const{
		if(x!=p.x) return x<p.x;
		if(y!=p.y) return y<p.y;
		if(z!=p.z)return z<p.z;
		return w<p.w;
		//return x != p.x ? x<p.x: y<p.y;
	}
	bool operator > (const pa4 &p) const{
		if(x!=p.x) return x>p.x;
		if(y!=p.y) return y>p.y;
		if(z!=p.z)return z>p.z;
		return w>p.w;
		//return x != p.x ? x<p.x: y<p.y;
	}
	bool operator == (const pa4 &p) const{
		return x==p.x && y==p.y && z==p.z &&w==p.w;
	}
		

};
class pa2{
	public:
	int x,y;
	pa2(int x=0,int y=0):x(x),y(y) {}
	pa2 operator + (pa2 p) {return pa2(x+p.x,y+p.y);}
	pa2 operator - (pa2 p) {return pa2(x-p.x,y-p.y);}
	bool operator < (const pa2 &p) const{
		return y != p.y ? y<p.y: x<p.x;
	}
	bool operator > (const pa2 &p) const{
		return x != p.x ? x<p.x: y<p.y;
	}
	bool operator == (const pa2 &p) const{
		return abs(x-p.x)==0 && abs(y-p.y)==0;
	}
	bool operator != (const pa2 &p) const{
		return !(abs(x-p.x)==0 && abs(y-p.y)==0);
	}
		

};



string itos( int i ) {
	ostringstream s ;
	s << i ;
	return s.str() ;
}
 
int Gcd(int v,int b){
	if(v==0) return b;
	if(b==0) return v;
	if(v>b) return Gcd(b,v);
	if(v==b) return b;
	if(b%v==0) return v;
	return Gcd(v,b%v);
}


 int mod;
int extgcd(int a, int b, int &x, int &y) {
    if (b == 0) {
        x = 1;
        y = 0;
        return a;
    }
    int d = extgcd(b, a%b, y, x);
    y -= a/b * x;
    return d;
}
pa operator+(const pa & l,const pa & r) {   
    return {l.first+r.first,l.second+r.second};                                    
}    
pa operator-(const pa & l,const pa & r) {   
    return {l.first-r.first,l.second-r.second};                                    
}  

ostream& operator<<(ostream& os, const vector<int>& VEC){
	for(auto v:VEC)os<<v<<" ";
    return os;
}

ostream& operator<<(ostream& os, const pa& PAI){
	os<<PAI.first<<" : "<<PAI.second;
    return os;
}

ostream& operator<<(ostream& os, const pa3& PAI){
	os<<PAI.x<<" : "<<PAI.y<<" : "<<PAI.z;
    return os;
}

ostream& operator<<(ostream& os, const pa4& PAI){
	os<<PAI.x<<" : "<<PAI.y<<" : "<<PAI.z<<" : "<<PAI.w;
    return os;
}




int beki(int wa,int rr,int warukazu){
	if(rr==0) return 1%warukazu;
	if(rr==1) return wa%warukazu;
	wa%=warukazu;
	if(rr%2==1) return ((ll)beki(wa,rr-1,warukazu)*(ll)wa)%warukazu;
	ll zx=beki(wa,rr/2,warukazu);
	return (zx*zx)%warukazu;
}

              
int pr[1100000];
int inv[1100000];



int comb(int nn,int rr){
	if(rr<0 || rr>nn || nn<0) return 0;
	int r=pr[nn]*inv[rr];
	r%=mod;
	r*=inv[nn-rr];
	r%=mod;
	return r;
}

void gya(int ert){
	pr[0]=1;
	for(int i=1;i<=ert;i++){
		pr[i]=((ll)pr[i-1]*i)%mod;
	}
		inv[ert]=beki(pr[ert],mod-2,mod);
	for(int i=ert-1;i>=0;i--){
		inv[i]=(ll)inv[i+1]*(i+1)%mod;
	}
}


                


                
              //   cin.tie(0);
    		//	ios::sync_with_stdio(false);
    			//priority_queue<pa3,vector<pa3>,greater<pa3>> pq;            
                 //sort(ve.begin(),ve.end(),greater<int>());
 //   mt19937(clock_per_sec);
  //  mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count()) ;

struct Segmax{
	//       1
	//   2        3
	// 4   5   6    7
	
	
	private:
	public:
	
	// (1<<15)=32768
	// 1<<16 = 65536
	// 1<<17 = 131072
	// 1<<18 = 262144
	
	int cor=(1<<19);
	
	vector<pa> vec;
	
	void shoki1(){
		vec.resize(2*cor+3, mp(0,0));
		for(int i=cor;i<2*cor;i++)vec[i].second=i-cor;
	}
	
	void shoki2(){
		for(int i=cor-1;i>0;i--) {
			if(vec[2*i].first>=vec[2*i+1].first) vec[i]=vec[2*i];
			else vec[i]=vec[2*i+1];
		}
	}
		
	void updadd(int x,int w){
		//x 項目に w加算
		x+=cor;
		vec[x].first+=w;
		x/=2;
		while(x){
			if(vec[2*x].first>=vec[2*x+1].first) vec[x]=vec[2*x];
			else vec[x]=vec[2*x+1];
			x/=2;
		}
	}
	
	void updchan(int x,int w){
		//x項目をwに変更
		x+=cor;
		vec[x].first=w;
		x/=2;
		while(x){
			if(vec[2*x].first>=vec[2*x+1].first) vec[x]=vec[2*x];
			else vec[x]=vec[2*x+1];
			x/=2;
		}
	}
	
	
	// [a,b)
	// k-th node
	// k no kukanha [l,r)
	pa segmax(int a,int b){
		a+=cor;
		b+=cor;
		pa ans=mp(1-(1ll<<31),-1);
		while(a<b){
			if(a&1){
				if(ans.first!=vec[a].first)ans=max(ans,vec[a]);
				else ans.second=min(ans.second,vec[a].second);
				a++;
			}
			if(b&1){
				b--;
				if(ans.first!=vec[b].first)ans=max(ans,vec[b]);
				else ans.second=min(ans.second,vec[b].second);
			}
			a/=2;
			b/=2;
		}
		return ans;
		
	}
	
	// 位置がp以上の要素で，値がx以上のもので最も左にあるもの
	// ない場合はcor
	int lower_bound(int p,int x){
		p+=cor;
		while(1){
			while(!(p&1))p/=2;
			if(vec[p].first>=x)break;//条件を満たさなくなる
			p++;
			if(p==(p&(-p))) return cor;
		}
		while(1){
			if(p>=cor) return p-cor;
			p*=2;
			if(vec[p].first<x){//条件を満たす
				p++;
			}
		}
	}

	
};

struct unionfind{
	private:
	public:
	
vector<int> par,ranks,kosuu;
	
	void shoki(int N){
		par.resize(N+1,0);
		ranks.resize(N+1,0);
		kosuu.resize(N+1,1);
		for(int i=0;i<=N;i++){
			par[i]=i;
		}
	}

	int root(int x){
		return par[x]==x ? x : par[x]=root(par[x]);
	}

	bool same(int x,int y){
		return root(x)==root(y);
	}
	bool is_root(int x){
		return x==root(x);
	}
	void unite(int x,int y){
 		x=root(x);
	 	y=root(y);
		int xx=kosuu[x],yy=kosuu[y];
	 	if(x==y) return;
		if(ranks[x]<ranks[y]){
			par[x]=y;
			kosuu[y]=yy+xx;
		}
	 	else {
			par[y]=x;
			if(ranks[x]==ranks[y]) ranks[x]=ranks[x]+1;
	 		kosuu[x]=yy+xx;
	 	}
		return;
	}
};

void solve(){
unionfind uf;
	int n,m;
	cin>>n>>m;
	uf.shoki(n+10);
	for(int i=0;i<m;i++){
		int y,yy;
		cin>>y>>yy;
		uf.unite(y,yy);
	}
	int ans=0;
	for(int i=1;i<=n;i++)if(i==uf.root(i))ans++;
	cout<<ans-1<<endl;
}
 
signed main(){
	
	cin.tie(0);
	ios::sync_with_stdio(false);
int n=1;
	//cin>>n;
 
	
	for(int i=0;i<n;i++)solve();
	
	return 0;
 }

Measured memory_bytes: 5608