#pragma region template
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ull = unsigned long long;
using ld = long double;
using vi = vector<int>;
using vvi = vector<vi>;
using vvvi = vector<vvi>;
using vll = vector<ll>;
using vvll = vector<vll>;
using vvvll = vector<vvll>;
using vld = vector<ld>;
using vvld = vector<vld>;
using vvvld = vector<vvld>;
using vs = vector<string>;
using pll = pair<ll, ll>;
using vp = vector<pll>;
template <typename T>
using pqrev = priority_queue<T, vector<T>, greater<T>>;
#define rep(i, n) for (ll i = 0, i##_end = (n); i < i##_end; i++)
#define repb(i, n) for (ll i = (n)-1; i >= 0; i--)
#define repr(i, a, b) for (ll i = (a), i##_end = (b); i < i##_end; i++)
#define reprb(i, a, b) for (ll i = (b)-1, i##_end = (a); i >= i##_end; i--)
#define ALL(a) (a).begin(), (a).end()
#define SZ(x) ((ll)(x).size())
//*
constexpr ll MOD = 1e9 + 7;
/*/
constexpr ll MOD = 998244353;
//*/
constexpr ll INF = 1e+18;
constexpr ld EPS = 1e-12L;
constexpr ld PI = 3.14159265358979323846L;
constexpr ll GCD(ll a, ll b) { return b ? GCD(b, a % b) : a; }
constexpr ll LCM(ll a, ll b) { return a / GCD(a, b) * b; }
template <typename S, typename T>
constexpr bool chmax(S &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <typename S, typename T>
constexpr bool chmin(S &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}
#ifdef OJ_LOCAL
#include "dump.hpp"
#else
#define dump(...) ((void)0)
#endif
template <typename T>
bool print_(const T &a) {
  cout << a;
  return true;
}
template <typename T>
bool print_(const vector<T> &vec) {
  for (auto &a : vec) {
    cout << a;
    if (&a != &vec.back()) {
      cout << " ";
    }
  }
  return false;
}
template <typename T>
bool print_(const vector<vector<T>> &vv) {
  for (auto &v : vv) {
    for (auto &a : v) {
      cout << a;
      if (&a != &v.back()) {
        cout << " ";
      }
    }
    if (&v != &vv.back()) {
      cout << "\n";
    }
  }
  return false;
}
void print() { cout << "\n"; }
template <typename Head, typename... Tail>
void print(Head &&head, Tail &&... tail) {
  bool f = print_(head);
  if (sizeof...(tail) != 0) {
    cout << (f ? " " : "\n");
  }
  print(forward<Tail>(tail)...);
}
#pragma endregion

void Pr(bool f){
  cout << (f ? "Yes" : "No") << "\n";
  exit(0);
}

// based on ACL
class UnionFind {
  int _n;
  // root node: -1 * component size
  // otherwise: parent
  vector<int> ps;
  int num_groups;

public:
  UnionFind() : _n(0) {}
  UnionFind(int n) : _n(n), ps(n, -1), num_groups(n) {}

  int unite(int a, int b) {
    assert(0 <= a && a < _n);
    assert(0 <= b && b < _n);
    int x = root(a), y = root(b);
    if (x == y) return x;
    if (-ps[x] < -ps[y]) swap(x, y);
    ps[x] += ps[y];
    ps[y] = x;
    num_groups--;
    return x;
  }

  bool same(int a, int b) {
    assert(0 <= a && a < _n);
    assert(0 <= b && b < _n);
    return root(a) == root(b);
  }

  int root(int a) {
    assert(0 <= a && a < _n);
    if (ps[a] < 0) return a;
    return ps[a] = root(ps[a]);
  }

  int size(int a) {
    assert(0 <= a && a < _n);
    return -ps[root(a)];
  }

  int getNum() { return num_groups; }

  // TBT sort O(n lon n)?
  vector<vector<int>> groups() {
    vector<int> leader_buf(_n), group_size(_n, 0);
    for (int i = 0; i < _n; i++) {
      leader_buf[i] = root(i);
      group_size[leader_buf[i]]++;
    }
    vector<vector<int>> result(_n);
    for (int i = 0; i < _n; i++) { result[i].reserve(group_size[i]); }
    for (int i = 0; i < _n; i++) { result[leader_buf[i]].emplace_back(i); }
    result.erase(remove_if(result.begin(), result.end(),
                           [](const vector<int> &v) { return v.empty(); }),
                 result.end());
    sort(result.begin(), result.end(),
         [](const vector<int> &a, const vector<int> &b) {
           return a.size() < b.size();
         });
    for (auto &&e : result) { sort(e.begin(), e.end()); }
    return result;
  }
};


int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(20);
  ll n, m;
  cin >> n >> m;
  UnionFind uf(n);
  rep(i, m){
    ll a, b;
    cin >> a >> b;
    a--; b--;
    uf.unite(a, b);
  }
  print(uf.getNum()-1);
}


Measured memory_bytes: 3748