#include <bits/stdc++.h>
using namespace std;
const double PI = 2 * acos(0.0);
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
mt19937_64 rng_64(chrono::steady_clock::now().time_since_epoch().count());
const string DIGITS = "0123456789";
const string ALPH = "abcdefghijklmnopqrstuvwxyz";
template <class T0, class T1>
inline ostream &operator<<(ostream &out, pair<T0, T1> &a) {
  return out << "{" << a.first << ", " << a.second << "}";
}
template <class T0, class T1, class T2>
inline ostream &operator<<(ostream &out, tuple<T0, T1, T2> &a) {
  return out << "{" << get<0>(a) << ", " << get<1>(a) << ", " << get<2>(a)
             << "}";
}
template <class T0, class T1, class T2, class T3>
inline ostream &operator<<(ostream &out, tuple<T0, T1, T2, T3> &a) {
  return out << "{" << get<0>(a) << ", " << get<1>(a) << ", " << get<2>(a)
             << ", " << get<3>(a) << "}";
}
template <class T>
inline ostream &operator<<(ostream &out, vector<T> &a) {
  out << "[";
  for (int i = 0; i < int(a.size()); ++i)
    out << a[i] << vector<string>{", ", "]  "}[i + 1 == a.size()];
  return out;
}
void smain();
signed main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  cout << setprecision(12) << fixed;
  smain();
  return 0;
}
const int N = 1e5 + 100;
int parent[N];
int sz[N];
vector<int> comp[N];
void make_set(int v) {
  parent[v] = v;
  sz[v] = 1;
}
int find_set(int v) {
  return parent[v] == v ? v : parent[v] = find_set(parent[v]);
}
void union_sets(int u, int v) {
  u = find_set(u);
  v = find_set(v);
  if (u == v) return;
  if (sz[u] < sz[v]) swap(u, v);
  parent[v] = u;
  sz[u] += sz[v];
}
vector<int> g[N];
void smain() {
  int n, m;
  cin >> n >> m;
  for (int i = 0; i < int(m); ++i) {
    int u, v;
    cin >> u >> v;
    g[u].push_back(v);
    g[v].push_back(u);
  }
  for (int i = 1; i <= n; ++i) make_set(i);
  int ans = 0;
  while (true) {
    vector<pair<pair<int, int>, int>> add;
    for (int v = 1; v <= n; ++v) {
      comp[v].clear();
    }
    for (int v = 1; v <= n; ++v) {
      comp[find_set(v)].push_back(v);
    }
    set<int> all;
    for (int i = 1; i <= n; ++i) all.insert(i);
    for (int v = 1; v <= n; ++v) {
      if (parent[v] != v) continue;
      for (auto x : comp[v]) {
        all.erase(x);
      }
      bool ok = false;
      for (auto x : comp[v]) {
        for (auto to : g[x]) {
          if (find_set(to) != v) {
            all.erase(to);
          }
        }
        if (!all.empty()) {
          ok = true;
          add.push_back({{x, *all.begin()}, 0});
        }
        for (auto to : g[x]) {
          if (find_set(to) != v) {
            all.insert(to);
          }
        }
        if (ok) break;
      }
      if (!ok) {
        if (!all.empty()) add.push_back({{v, *all.begin()}, 1});
      }
      for (auto x : comp[v]) {
        all.insert(x);
      }
    }
    bool have = false;
    for (auto x : add) {
      if (find_set(x.first.first) != find_set(x.first.second)) {
        have = true;
        ans += x.second;
        union_sets(x.first.first, x.first.second);
      }
    }
    if (!have) break;
  }
  cout << ans;
}
