#include <bits/stdc++.h>
using namespace std;
const int inf = 0x3f3f3f3f;
const long long INF = 0x3f3f3f3f3f3f3f3fLL;
const double PI = acos(-1.0);
const long double eps = 1e-14;
const int mod = 1e9 + 7;
const int maxn = 20;
const int N = 1e5 + 10;
const int M = 1e9 + 7;
const long long mm = (1LL << 32);
template <class T>
inline void read(T &x) {
  x = 0;
  char c = getchar();
  bool f = 0;
  for (; !isdigit(c); c = getchar()) f ^= c == '-';
  for (; isdigit(c); c = getchar()) x = x * 10 + (c ^ 48);
  x = f ? -x : x;
}
template <class T>
inline void write(T x) {
  if (x < 0) {
    putchar('-');
    x = -x;
  }
  T y = 1;
  int len = 1;
  for (; y <= x / 10; y *= 10) ++len;
  for (; len; --len, x %= y, y /= 10) putchar(x / y + 48);
}
long long qpow(long long a, long long b, long long MOD) {
  long long ans = 1;
  while (b) {
    if (b & 1) ans = (ans * a) % MOD;
    b >>= 1;
    a = (a * a) % MOD;
  }
  return ans;
}
set<int> G[N], s;
int n, m, vis[N];
void bfs(int x) {
  queue<int> q;
  s.erase(x);
  q.push(x);
  vis[x] = 1;
  while (!q.empty()) {
    int k = q.front();
    q.pop();
    for (auto it = s.begin(); it != s.end();) {
      int v = *it++;
      if (G[v].count(k) == 0) {
        s.erase(v);
        q.push(v);
        vis[v] = 1;
      }
    }
  }
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cin >> n >> m;
  for (int i = 1; i <= n; ++i) s.insert(i);
  for (int i = 0; i < m; ++i) {
    int x, y;
    cin >> x >> y;
    G[x].insert(y);
    G[y].insert(x);
  }
  int cnt = 0;
  for (int i = 1; i <= n; ++i) {
    if (!vis[i]) {
      cnt++;
      bfs(i);
    }
  }
  cout << cnt - 1 << endl;
  return 0;
}
