#include <bits/stdc++.h>
using namespace std;
const int maxn = 100005, maxm = 2000005, maxq = maxn;
int n, m, head[maxn], cnt, pre[maxn], suf[maxn], q[maxq], tot, size[maxn];
bool vis[maxn];
struct _edge {
  int v, next;
} g[maxm << 1];
inline int iread() {
  int f = 1, x = 0;
  char ch = getchar();
  for (; ch < '0' || ch > '9'; ch = getchar()) f = ch == '-' ? -1 : 1;
  for (; ch >= '0' && ch <= '9'; ch = getchar()) x = x * 10 + ch - '0';
  return f * x;
}
inline void add(int u, int v) {
  g[cnt] = (_edge){v, head[u]};
  head[u] = cnt++;
}
inline void del(int x) {
  int t = pre[x];
  suf[t] = suf[x];
  pre[suf[x]] = t;
}
inline void bfs(int x) {
  int h = 0, t = 0;
  q[t++] = x;
  while (h != t) {
    size[tot]++;
    int u = q[h++];
    for (int i = head[u]; ~i; i = g[i].next) vis[g[i].v] = 1;
    for (int i = suf[0]; i; i = suf[i])
      if (!vis[i]) del(i), q[t++] = i;
    for (int i = head[u]; ~i; i = g[i].next) vis[g[i].v] = 0;
  }
}
int main() {
  n = iread();
  m = iread();
  for (int i = 1; i <= n; i++) head[i] = -1;
  cnt = 0;
  for (int i = 1; i <= m; i++) {
    int x = iread(), y = iread();
    add(x, y);
    add(y, x);
  }
  for (int i = 0; i <= n; i++) pre[i] = i - 1, suf[i] = i + 1;
  pre[0] = suf[n] = 0;
  for (int i = suf[0]; i; i = suf[0]) {
    del(i);
    tot++;
    bfs(i);
  }
  sort(size + 1, size + 1 + tot);
  printf("%d", tot - 1);
  return 0;
}
