#include <bits/stdc++.h>
using namespace std;
const long long MOD = (long long)1e9 + 7;
const long long N = (long long)2e6 + 222;
const long long INF = (long long)1e18;
const double eps = (double)1e-6;
long long n, m, l, r, used[N];
vector<long long> g[N];
set<long long> s;
long long cnt = 0;
void dfs(long long x) {
  used[x] = 1;
  s.erase(x);
  cnt++;
  long long pr = 0;
  for (auto to : g[x]) {
    while (s.upper_bound(pr) != s.end() && (*s.upper_bound(pr)) < to) {
      long long k = (*s.upper_bound(pr));
      if (!used[k]) dfs(k);
    }
    pr = to;
  }
}
void solve() {
  cin >> n >> m;
  for (long long i = 1; i <= m; ++i) {
    cin >> l >> r;
    g[l].push_back(r);
    g[r].push_back(l);
  }
  for (long long i = 1; i <= n; ++i) s.insert(i);
  long long ans = 0;
  for (long long i = 1; i <= n; ++i) sort(g[i].begin(), g[i].end());
  for (long long i = 1; i <= n; ++i) g[i].push_back(n + 1);
  for (long long i = 1; i <= n; ++i) {
    if (!used[i]) {
      ans++;
      dfs(i);
    }
  }
  cout << ans - 1;
}
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long long t = 1;
  while (t--) {
    solve();
  }
  return 0;
}
