#pragma GCC optimize ("O3")
#include <iostream>
#include <iomanip>
#include <istream>
#include <ostream>
#include <sstream>
#include <iterator>
#include <vector>
#include <algorithm>
#include <queue>
#include <deque>
#include <list>
#include <stack>
#include <map>
#include <unordered_map>
#include <set>
#include <bitset>
#include <utility>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <string>
#include <ctime>
#include <cctype>
#include <cstdlib>
#include <numeric>
#define IINF 1000000000
#define INF 3223372036854775807
#define MOD 1000000007
#define mod 998244353
#define INT_MAX_ 2147483647
#define EPS (1e-10)
#define REP(i, a, n) fo-r (ll i = a; i < (ll)(n); i++)
#define REPE(i, a, n) for (ll i = a; i <= (ll)(n); i++)
//#define rep(i,n)for (ll i = 0; i < (ll)(n); i++)
#define rep(i,l,r)for(ll i=(l);i<(r);i++)
#define rep1(i,n) for(int i=1;i<=(int)(n);i++)
#define Endl endl
#define fi first
#define se second
#define pb push_back
#define mp make_pair
#define mt make_tuple
#define eb emplace_back
#define mmax(x,y)(x>y?x:y)
#define mmin(x,y)(x<y?x:y)
#define chmax(x,y) x=mmax(x,y)
#define chmin(x,y) x=mmin(x,y)
#define all(x) (x).begin(),(x).end()
#define siz(x) (ll)(x).size()
#define PI acos(-1.0)
#define me memset
#define bit(n,k) ((n>>k)&1)
#define lg length()
using namespace std;
typedef long long int ll;
typedef unsigned long long int ull;
typedef long double ld;
typedef pair<int,int>Pin;
typedef pair<ll,ll>Pll;
template<class T> using V=vector<T>;
template<typename T> using min_priority_queue = priority_queue<T, vector<T>, greater<T> >;
long long GCD(long long a, long long b) {return b?GCD(b,a%b):a;}
long long LCM(long long a, long long b) {return a/GCD(a,b)*b;}
ll pom(ll a,ll n,int m){ll x=1;for(a%=m;n;n/=2)n&1?x=x*a%m:0,a=a*a%m;return x;}
#define invp(a,p)pom(a,p-2,p)
int dx[4]={-1,0,1,0};
int dy[4]={0,-1,0,1};
int ddx[8]={-1,0,1,0,1,1,-1,-1};
int ddy[8]={0,-1,0,1,1,-1,1,-1};
ll cmp1(pair<Pll,ll> a,pair<Pll,ll> b){
        return a.fi.se>b.fi.se;
}
ll cmp2(pair<ll,ll> a,pair<ll,ll> b){
    if(a.fi!=b.fi)
        return  a.se<b.se;
    else
        return a.se>b.se;
}
//----------------------------------------------------------------------
struct unionfind{
    vector<int> rank,parent,size;
    unionfind(int n){
        for(int i=0;i<n;i++){
            rank.push_back(1);//树的高度
            parent.push_back(i);//根
            size.push_back(1);//树大小
        }
    }
    int getroot(int x){
        if(parent[x]==x)
            return x;
        return parent[x]=getroot(parent[x]);//押平路径加速以后的查找
    }
    bool issame(int a,int b){
        return getroot(a)==getroot(b);
    }
    void unite(int a,int b){
        a=getroot(a);
        b=getroot(b);
        if(a==b)
            return;
        else if(rank[a]>rank[b]){ //合并到高的树下
            parent[b]=a;
            size[a]+=size[b];
        }
        else if(rank[a]==rank[b]){
            parent[b]=a;
            rank[a]++;
            size[a]+=size[b];
        }
        else{
            parent[a]=b;
            size[b]+=size[a];
        }
    }
};
//----------------------------------------------------------------------
int main(int argc, char * argv[]){
    cin.tie(0);
    ios::sync_with_stdio(false);
    //-------------------------------  
    //ll begin_t=clock();
    //freopen("big.txt", "r", stdin);
    //freopen("out3.txt", "w", stdout);
    //------------------------------
    ll n,m;cin>>n>>m;
    unionfind uf(n);
    bool abc=0;
    ll root = -1;
    for(ll i=0;i<m;i++){
        ll a,b;cin>>a>>b;
        a--;b--;
        uf.unite(a,b);
        if(abc==0){
            abc=1;
            root=uf.getroot(a);
        }
    }
    ll ans=0;
    for(ll i=0;i<n;i++){
        if(!uf.issame(i,root)){
            uf.unite(root,i);
            ans++;
        }
    }
    cout<<ans<<Endl;
    //------------------------------
    //fclose(stdin);
    //fclose(stdout);
    //ll end_t=clock();cout<<"time="<<end_t-begin_t<<"ms"<<endl;
    //------------------------------- 
    return 0;
}
//----------------------------------------------------------------------








































Measured memory_bytes: 4544