#include<bits/stdc++.h>
#include<queue>
using namespace std;

#define what_is(x) cout << #x << " is " << x << endl;
#define PI acos(-1)
#define hell 1000000007
#define HELL 998244353
#define io ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0)
#define fix(n) cout << fixed << setprecision(n)
#define mset(a,n) memset(a,n,sizeof a)
#define rep(i,a,b) for (__typeof((b)) i=(a);i<(b);i++)
#define repp(i,a,b,p) for(__typeof((b)) i=(a);i<(b);i+=p)
#define ren(i,a,b) for(__typeof((a)) i=(a);i>=(b);i--)
#define renn(i,a,b,p) for(__typeof((a) i=(a);i>=(b);i-=p)
#define ADD(a,b,c) ((a)%c+(b)%c)%c
#define SUB(a,b,c) ((a)%c-(b)%c+c)%c
#define MUL(a,b,c) (((a)%c)*((b)%c))%c
#define lbd lower_bound
#define ubd upper_bound
#define ll long long
#define ld long double
#define pb push_back
#define fi first
#define se second
#define vll vector<ll>
#define pll pair<ll,ll>
#define vpll vector<pll>
#define all(v) (v).begin(), (v).end()
#define sz(x) (ll)x.size()
// #define out(n) cout<<n<<" "
#define outl(n) cout<<n<<endl
#define bug(n) {outl(n);return;}
#define pii pair<int,int>
#define lim 9223372036854775806
#define ninf -9223372036854775802
#define M 998244353  
#define eps 1e-9
#define MAX 2
#define N 2
#define db(a) for(auto it:a) {cout<<it<<" ";}cout<<endl;
ll prime[MAX];
ll fact[MAX];

const long long INF64 = 1e18;
ll vpow(ll a, ll b, ll p) {
    if (b == 0)
        return 1;
    if (b == 1)
        return a;
    ll temp = vpow(a, b / 2, p);
    temp *= temp;
    temp %= p;
    if (b % 2 == 0)
        return temp;
    temp = temp * a;
    temp %= p;
    return temp;
}

ll gcd(ll a, ll b) {
    if (a == 0)
        return b;
    return gcd(b % a, a);
}


ll inverseMod(ll a, ll b, ll p) {
    if (b == 0)
        return 1;
    ll temp = vpow(b, p - 2, p);
    ll ans = temp * a;
    ans %= p;
    return ans;
}
void sieve()
{
    prime[0] = prime[1] = 1;
    for (ll i = 2; i < MAX; i++) {
        if (prime[i] == 0) {
            for (int j = i * 2; j < MAX; j += i) {
                if (prime[j] == 0) {
                    prime[j] = i;
                }
            }
            prime[i] = i;
        }
    }
}


void factorial() {
    fact[0] = 1;
    fact[1] = 1;
    rep(i, 2, MAX) {
        fact[i] = (fact[i - 1] * i);
         fact[i]%=M;
    }
}
void dfs(vector<ll> adj[],ll u,vector<bool> &vis){
    vis[u]=true;
    //cout<<"IN "<<u<<endl;
    for(auto ch:adj[u]){
        if(!vis[ch])
            dfs(adj,ch,vis);
    }
}
void solve(ll test){
    ll n,m;
    cin>>n>>m;
    vll adj[n];
    //what_is(n);
    //what_is(m);
    rep(i,0,m){
        ll u,v;
        cin>>u>>v;
        u--;v--;
        //what_is(u);
        //what_is(v);
        adj[u].pb(v);
        adj[v].pb(u);
    }
    ll ans=0;
    vector<bool> vis(n,false);
    rep(i,0,n){
        if(!vis[i]){
			//cout<<"Not vis "<<i<<endl;
            ans++;
            dfs(adj,i,vis);
        }
    }
    if(ans>=1)
		ans--;
    cout<<ans<<endl;
}
int main() {
    io;
    // sieve();
    factorial();
#ifndef ONLINE_JUDGE
    freopen("input1.txt", "r", stdin);
    // freopen("output1.txt","w",stdout);
#endif

    ll t = 1;
    //cin >> t;
    fix(12);
    rep(q, 1, t + 1) {
        // cout<<"Test case : "<<q<<endl;
        solve(q);
        //solve2();
    }
    return 0;

}


Measured memory_bytes: 10756