#include <bits/stdc++.h>

#define ll long long
#define ld long double

#define rep(i, n) for(ll i = 0; i < n; ++i)
#define rep2(i, a, b) for(ll i = a; i <= b; ++i)
#define rrep(i, a, b) for(ll i = a; i >= b; --i)

#define pii pair<int, int>
#define pll pair<ll, ll>

#define fi first
#define se second

#define pb push_back
#define eb emplace_back

#define vi vector<int>
#define vll vector<ll>
#define vpii vector<pii>
#define vpll vector<pll>

#define all(a) a.begin(), a.end()

#define endl '\n'
using namespace std;

template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return true; } return false; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return true; } return false; }

template<uint_fast64_t Modulus>
struct modint {
    using u64 = uint_fast64_t;
    u64 a;
    constexpr modint(const u64 x = 0) noexcept : a(x % Modulus) {}
    constexpr u64& value() noexcept { return a; }
    constexpr const u64& value() const noexcept { return a; }
    constexpr modint operator+(const modint rhs) const noexcept {
        return modint(*this) += rhs;
    }
    constexpr modint operator-(const modint rhs) const noexcept {
        return modint(*this) -= rhs;
    }
    constexpr modint operator*(const modint rhs) const noexcept {
        return modint(*this) *= rhs;
    }
    constexpr modint operator/(const modint rhs) const noexcept {
        return modint(*this) /= rhs;
    }
    constexpr modint& operator+=(const modint rhs) noexcept {
        a += rhs.a;
        if (a >= Modulus) a -= Modulus;
        return *this;
    }
    constexpr modint& operator-=(const modint rhs) noexcept {
        if (a < rhs.a) a += Modulus;
        a -= rhs.a;
        return *this;
    }
    constexpr modint& operator*=(const modint rhs) noexcept {
        a = a * rhs.a % Modulus;
        return *this;
    }
    constexpr modint& operator/=(modint rhs) noexcept {
        return *this *= rhs.inv();
    }
    constexpr modint& operator++() noexcept {
        a++;
        if (a == Modulus) a = 0;
        return *this;
    }
    constexpr modint operator++(int) noexcept {
        modint tmp(*this);
        operator++();
        return tmp;
    }
    constexpr modint& operator--() noexcept {
        if (a == 0) a = Modulus;
        a--;
        return *this;
    }
    constexpr modint operator--(int) noexcept {
        modint tmp(*this);
        operator--();
        return tmp;
    }
    constexpr modint pow(u64 exp) const noexcept {
        modint res(1), mul(*this);
        for (; exp; mul *= mul, exp >>= 1) if (exp & 1) res *= mul;
        return res;
    }
    constexpr modint inv() const noexcept {
        return modint(*this).pow(Modulus - 2);
    }
    constexpr bool operator==(modint rhs) const noexcept { return a == rhs.a; }
    constexpr bool operator!=(modint rhs) const noexcept { return a != rhs.a; }
    friend ostream& operator<<(ostream& os, const modint<Modulus>& rhs) { return os << rhs.a; }
};

using mint = modint<10000007>;
// 返り値: a と b の最大公約数
// ax + by = gcd(a, b) を満たす (x, y) が格納される
long long extgcd(long long a, long long b, long long &x, long long &y) {
    if (b == 0) {
        x = 1;
        y = 0;
        return a;
    }
    long long d = extgcd(b, a%b, y, x);
    y -= a/b * x;
    return d;
}
const ll MOD=10000007;//998244353
const ll INF=1e18;
const int inf=1e9;
const double PI=acos(-1);
int dx[8] = {1,0,-1,0,1,1,-1,-1};
int dy[8] = {0,1,0,-1,-1,1,1,-1};
 
const int MAX=1e6+10;
class UnionFind {
public:
    vector <ll> par; // 各元の親を表す配列
    vector <ll> siz; // 素集合のサイズを表す配列(1 で初期化)

    // Constructor
    UnionFind(ll sz_): par(sz_), siz(sz_, 1LL) {
        for (ll i = 0; i < sz_; ++i) par[i] = i; // 初期では親は自分自身
    }
    void init(ll sz_) {
        par.resize(sz_);
        siz.assign(sz_, 1LL);  // resize だとなぜか初期化されなかった
        for (ll i = 0; i < sz_; ++i) par[i] = i; // 初期では親は自分自身
    }

    // Member Function
    // Find
    ll root(ll x) { // 根の検索
        while (par[x] != x) {
            x = par[x] = par[par[x]]; // x の親の親を x の親とする
        }
        return x;
    }

    // Union(Unite, Merge)
    bool merge(ll x, ll y) {
        x = root(x);
        y = root(y);
        if (x == y) return false;
        // merge technique（データ構造をマージするテク．小を大にくっつける）
        if (siz[x] < siz[y]) swap(x, y);
        siz[x] += siz[y];
        par[y] = x;
        return true;
    }

    bool issame(ll x, ll y) { // 連結判定
        return root(x) == root(y);
    }

    ll size(ll x) { // 素集合のサイズ
        return siz[root(x)];
    }
};
int main(){
  cin.tie(0);
  ios::sync_with_stdio(false); 
   
  int n,m;
  cin >> n >> m;
  UnionFind tree(n);
  
  rep(i,m){
    int a,b;
    cin >> a >> b;
    a--;
    b--;
    tree.merge(a,b);
  }
  map<int,int>mp;
  
  rep(i,n) mp[tree.root(i)]++;
  
  cout << mp.size()-1 << endl;
    

  return 0;
}

Measured memory_bytes: 8372