#include<bits/stdc++.h>
using namespace std;
typedef long long int ll;
#define mod 998244353
#define inf 9000000000000000000
typedef pair<ll,ll> pll;
typedef pair<int,int> pint;
#define PI 3.14159265359
#define endl '\n'
#define itn int
#define cout1(a) cout<<a<<endl
#define cout2(a,b) cout<<a<<' '<<b<<endl
#define cout3(a,b,c) cout<<a<<" "<<b<<" "<<c<<endl
#define cout4(a,b,c,d) cout<<a<<" "<<b<<" "<<c<<" "<<d<<endl
typedef priority_queue<pair<pll,pll>,vector<pair<pll,pll> >,greater<pair<pll,pll> > > pqset;
typedef priority_queue<pll,vector<pll>,greater<pll> > pqset_ll;
#define pb push_back
#define pf push_front
#define fi first
#define mkp make_pair
#define se second
#include<ext/pb_ds/assoc_container.hpp>
#include<ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
typedef tree<int,null_type,less<int>,rb_tree_tag,tree_order_statistics_node_update> os;
typedef tree<pll,null_type,less<pll>,rb_tree_tag,tree_order_statistics_node_update> os_pair;
#define rep(i,n) for(ll i=0;i<(ll)n;i++)
#define repeat(i,start,n) for(ll i=(ll)start;i<(ll)n;i++)
#define FileIO freopen("input.txt","r",stdin);freopen("output.txt","w",stdout)
ll max(int a,ll b){ return max((ll)a,b);}
ll min(int a,ll b){ return min((ll)a,b);}
ll min(ll a,int b){ return min(a,(ll)b);}
ll max(ll a,int b){ return max(a,(ll)b);}
ll power(ll a,ll b){
	if(a==1||b==0)	return 1;
	ll c=power(a,b/2);
	ll res=1;
	res=c*c;
	if(res>=mod)	res%=mod;
	if(b%2)	res*=a;
	if(res>=mod)	res%=mod;
	return res;
}
ll power(ll a,ll b,ll mod1){
	if(a==1||b==0)	return 1;
	ll c=power(a,b/2,mod1);
	ll res=1;
	res=c*c;
	if(res>=mod1)	res%=mod1;
	if(b%2)	res*=a;
	if(res>=mod1)	res%=mod1;
	return res;
}
ll modInv(ll a){
	return power(a,mod-2);
}
ll fact[1],inv[1];
void factorial(ll n){
	fact[0]=1;
	for(ll i=1;i<=n;i++){
		fact[i]=fact[i-1]*i;
		if(fact[i]>=mod)	fact[i]%=mod;
	}
}
void InvFactorial(ll n){
	inv[0]=1;
	for(ll i=1;i<=n;i++)	inv[i]=modInv(fact[i]);
}
ll ncr(ll n,ll r){
	if(n<r||n<0||r<0)	return 0;
	ll b=inv[n-r];
	ll c=inv[r];
	ll a=fact[n]*b;
	if(a>=mod)	a%=mod;
	a*=c;
	if(a>=mod)	a%=mod;
	return a;
}
bool prime[1];
vector<ll> primes;
void remove_duplicates(vector<pair<int,pint> > &v){
	sort(v.begin(),v.end());
	int _size=unique(v.begin(),v.end())-v.begin();
	v.resize(_size);
}
unsigned int gcd(unsigned int u, unsigned int v){
    if(u==0||v==0) return max(u,v);
    unsigned int shift=__builtin_ctz(u|v);
	u>>=__builtin_ctz(u);
    do{
    	v>>=__builtin_ctz(v);
    	if(u>v)	swap(u,v);
    	v-=u;
    }while(v!=0);
    return u<<shift;
}
void sieve(ll n){
    memset(prime,true,sizeof(prime));
	prime[1]=false;
    for(ll p=2;p*p<=n;p++)	if(prime[p])	for(ll i=p*p;i<=n;i+=p)	prime[i]=false;
    repeat(i,2,n+1)	if(prime[i])	primes.pb(i);
}
struct custom_hash{// Credits: https://codeforces.com/blog/entry/62393
    static uint64_t splitmix64(uint64_t x){
        // http://xorshift.di.unimi.it/splitmix64.c
        x+=0x9e3779b97f4a7c15;
    	x=(x^(x>>30))*0xbf58476d1ce4e5b9;
        x=(x^(x>>27))*0x94d049bb133111eb;
        return x^(x>>31);
    }
    size_t operator()(uint64_t x)const{
        static const uint64_t FIXED_RANDOM = chrono::steady_clock::now().time_since_epoch().count();
        return splitmix64(x+FIXED_RANDOM);
    }
    size_t operator()(pair<int64_t,int64_t> Y) const{
        static const uint64_t FIXED_RANDOM = chrono::steady_clock::now().time_since_epoch().count();
        return splitmix64(Y.first*31+Y.second+FIXED_RANDOM);
    }
};
//Use(<<)for multiplication
//Use(>>)for division
//find_by_order -> value at index
//order_of_key -> index of value
//in Floyd-Warshall Algo, k is outer loop
//__gcd(0,0) gives runtime error
//power(a,b) == power(a,b%phi(a)) Fermet's Theorem
//never use "=" operator in compare function
//For checking odd cycles, check bipartite
//Range in iterative segment tree [a,b)
//no of prime numbers in range : (70,19) , (1000,168) , (100000,1229) , (sqrt(10^9),3409) ;
//use external for-loop(in driver function) to avoid memory limit exceeded in top-down dp
vector<int> v[200001];
int visited[200001];
void dfs(int i){
	visited[i]=1;
	for(int j:v[i]){
		if(visited[j])	continue;
		dfs(j);
	}
}
void solve(){
	int n,m;
	cin>>n>>m;
	rep(i,m){
		int a,b;
		cin>>a>>b;
		v[a].pb(b);
		v[b].pb(a);
	}
	int count=0;
	repeat(i,1,n+1){
		if(visited[i])	continue;
		dfs(i);
		count++;
	}
	cout1(count-1);
}
int main(){
	//freopen("input.txt","r",stdin);freopen("output.txt","w",stdout);
	cin.tie(NULL);cout.tie(NULL);cin.sync_with_stdio(0);cout.sync_with_stdio(0);cout<<fixed;
	srand(time(0));
	//cout.precision(9);
	int t=1;
	//cin>>t;
	int countu=1;
	for(int i=1;i<=t;i++){
		//cout<<"Case #"<<i<<": ";
		solve();
	}
}

Measured memory_bytes: 12360