#include <bits/stdc++.h>
using namespace std;
template <class C>
void min_self(C &a, C b) {
  a = min(a, b);
}
template <class C>
void max_self(C &a, C b) {
  a = max(a, b);
}
long long mod(long long n, long long m = 1000000007) {
  n %= m, n += m, n %= m;
  return n;
}
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
struct custom_hash {
  static uint64_t splitmix64(uint64_t x) {
    x += 0x9e3779b97f4a7c15;
    x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
    x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
    return x ^ (x >> 31);
  }
  size_t operator()(uint64_t x) const {
    static const uint64_t FIXED_RANDOM =
        chrono::steady_clock::now().time_since_epoch().count();
    return splitmix64(x + FIXED_RANDOM);
  }
};
unordered_map<long long, int, custom_hash> safe_map;
const int MAXN = 1e5 + 5;
const int LOGN = 21;
const long long INF = 1e14;
int dx[] = {1, 0, -1, 0};
int dy[] = {0, 1, 0, -1};
set<int> gr[MAXN];
set<int> vertices;
vector<int> comps;
int cc = 0;
void dfs(int node) {
  vertices.erase(node);
  comps[cc]++;
  int now = -1;
  while (1) {
    auto next = vertices.upper_bound(now);
    if (next == vertices.end()) break;
    now = *next;
    if (gr[node].count(now)) continue;
    dfs(now);
  }
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long n, m;
  cin >> n >> m;
  for (int i = 0; i < m; i++) {
    int u, v;
    cin >> u >> v;
    u--, v--;
    gr[u].insert(v);
    gr[v].insert(u);
  }
  for (int i = 0; i < n; i++) {
    vertices.insert(i);
  }
  for (int i = 0; i < n; i++) {
    if (vertices.count(i)) {
      comps.push_back(0);
      dfs(i);
      cc++;
    }
  }
  int ans = comps.size() - 1;
  cout << ans, cout << "\n";
  cerr << "\nTime elapsed: " << 1000 * clock() / CLOCKS_PER_SEC << "ms\n";
  return 0;
}
