#include <bits/stdc++.h>
#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
using namespace std;
/*----------------------------------ここからマクロ----------------------------------*/
#define all(a) (a).begin(),(a).end()
#define rall(a) (a).rbegin(),(a).rend()
#define vecin(a) rep(i,a.size())cin >> a[i]
#define overload4(_1,_2,_3,_4,name,...) name
/*#define rep1(n) for(int i=0;i<(int)n;++i)
#define rep2(i,n) for(int i=0;i<(int)n;++i)
#define rep3(i,a,b) for(int i=(int)a;i<(int)b;++i)
#define rep4(i,a,b,c) for(int i=(int)a;i<(int)b;i+=(int)c)*/
#define rep1(n) for(ll i=0;i<(ll)n;++i)
#define rep2(i,n) for(ll i=0;i<(ll)n;++i)
#define rep3(i,a,b) for(ll i=(ll)a;i<(ll)b;++i)
#define rep4(i,a,b,c) for(int i=(ll)a;i<(ll)b;i+=(ll)c)
#define rep(...) overload4(__VA_ARGS__,rep4,rep3,rep2,rep1)(__VA_ARGS__)
#ifdef _DEBUG
#define debug1(a) cerr << #a << ": " << a << "\n"
#define debug2(a,b) cerr << #a << ": " << a << ", " << #b << ": " << b << "\n"
#define debug3(a,b,c) cerr << #a << ": " << a << ", " << #b << ": " << b << ", " << #c << ": " << c << "\n"
#define debug4(a,b,c,d) cerr << #a << ": " << a << ", " << #b << ": " << b << ", " << #c << ": " << c << ", " << #d << ": " << d << "\n"
#define debug(...) overload4(__VA_ARGS__,debug4,debug3,debug2,debug1)(__VA_ARGS__)
#define vecout(a) cerr << #a << ": [";rep(i,a.size()){cerr << a[i];cerr << (i == a.size() - 1 ? "":",");}cerr << "]\n"
#else
#define debug(...)
#define vecout(a)
#endif
#define mp make_pair
//struct doset{doset(int n){cout << fixed << setprecision(n);cerr << fixed << setprecision(n);}};
//struct myset{myset(){ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);}};
void myset(){ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);}
void doset(int n){cout << fixed << setprecision(n);}
using ll = long long;
using ld = long double;
using dou = double;
const int inf = 1 << 30;
const ll INF = 1LL << 60;
const ld pi = 3.14159265358;
const ll mod1 = 1000000007LL;
const ll mod2 = 998244353LL;
typedef pair<ll,ll> P;
template<class T, class U> inline bool chmin(T& a, const U& b){ if(a > b){ a = b; return 1; } return 0; }
template<class T, class U> inline bool chmax(T& a, const U& b){ if(a < b){ a = b; return 1; } return 0; }
//nのm乗をMODで割ったあまりO(logm)
ll modpow(ll n,ll m,ll MOD){
    if(m == 0)return 1;
    if(m < 0)return 0;
    ll res = 1;
    n %= MOD;
    while(m){
        if(m & 1)res = (res * n) % MOD;
        m >>= 1;
        n *= n;
        n %= MOD;
    }
    return res;
}

ll mypow(ll n,ll m){
    if(m == 0)return 1;
    if(m < 0)return -1;
    ll res = 1;
    while(m){
        if(m & 1)res = (res * n);
        m >>= 1;
        n *= n;
    }
    return res;
}

//素数判定O(sqrt(N))
template<class T>
inline bool isp(T n){
    bool res = true;
    if(n == 1 || n == 0)return false;
    else{
        for(ll i = 2;i * i <= n;i++){
            if(n % i == 0){
                res = false;
                break;
            }
        }
        return res;
    }
}


inline bool Yes(bool b = 1){cout << (b ? "Yes\n":"No\n");return b;}
inline bool YES(bool b = 1){cout << (b ? "YES\n":"NO\n");return b;}


map<ll,ll> primefactor(ll n){
    map<ll,ll> ma;
    if(n <= 1)return ma;
    ll m = n;
    for(ll i = 2;i * i <= n;i++){
        while(m % i == 0){
            ma[i]++;
            m /= i;
        }
    }
    if(m != 1)ma[m]++;
    return ma;
}

ll __lcm(ll a,ll b){
    return a / __gcd(a,b) * b;
}


template<ll MOD = mod1>
struct fac_solver{
private:
    ll cmax;
    vector<ll> fac,finv,inv;
public:
    fac_solver(ll n = 100000):cmax(n),fac(n),finv(n),inv(n){
        fac[0] = fac[1] = 1;
        finv[0] = finv[1] = 1;
        inv[1] = 1;
        for (ll i = 2; i < cmax; i++){
            fac[i] = fac[i - 1] * i % MOD;
            inv[i] = MOD - inv[MOD%i] * (MOD / i) % MOD;
            finv[i] = finv[i - 1] * inv[i] % MOD;
        }
    }

    //二項係数計算nCk
    ll nCk(ll n, ll k){
        if (n < k) return 0;
        if (n < 0 || k < 0) return 0;
        return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
    }

    ll nPk(ll n,ll k){
        if(n < k || n < 0 || k < 0)return 0;
        return fac[n] * finv[n - k] % MOD;
    }

    ll stair_pow(ll n){return fac[n];}
};



/*----------------------------------マクロここまで----------------------------------*/

template<class T>
struct UnionFind{
    vector<T> par;
    vector<T> ran;
    vector<T> siz;
    T tree_num;

    UnionFind(T n) : par(n),ran(n,0),siz(n,1),tree_num(n){
        rep(i,n){
            par[i] = i;
        }
    }
    T find(T x){
        if(x == par[x])return x;
        else return par[x] = find(par[x]);
    }
    void unite(T x,T y){
        x = find(x);
        y = find(y);
        if(x == y)return;
        tree_num--;
        if(ran[x] < ran[y]){
            siz[y] += siz[x];
            par[x] = y;
        }
        else{
            siz[x] += siz[y];
            par[y] = x;
            if(ran[x] == ran[y])ran[x]++;
        }
    }
    bool same(T x,T y){
        return find(x) == find(y);
    }
    T get_size(T x){
        return siz[find(x)];
    }
    T renketsu_seibun(){
        return tree_num;
    }
};


int main(){
    myset();
    ll N,M;
    cin >> N >> M;
    UnionFind<ll> UF(N);
    rep(i,M){
        ll a,b;
        cin >> a >> b;
        a--;b--;
        UF.unite(a,b);
    }
    cout << UF.tree_num - 1 << "\n";
}


Measured memory_bytes: 5560