#include <bits/stdc++.h>
#include <bits/extc++.h>
#define int long long
char freadgetchar(){
	static char buf[100000],*p1=buf,*p2=buf;
	if(p1==p2){
		p2=(p1=buf)+fread(buf,1,100000,stdin);
		if (p1==p2)return EOF;
	}
	return *p1++;
}
void read(int&x){
	static char c;
	c=freadgetchar();
	int b=1;
	for(x = 0;!(c>='0'&&c<='9');c=freadgetchar()) if(c == '-') b = -b;
	for(;c>='0'&&c<='9';x=x*10+c-'0',c=freadgetchar());x*=b;
}
const int MAXN=1e6+5;
std::vector<int> g[MAXN];
int n,m;
int tag[MAXN],vis[MAXN],s[MAXN];
int dgr[MAXN];
int fa[MAXN],tot;
void init(){
	for(int i=1;i<=n;i++)fa[i]=i;
}
int findfa(int x){
	if(x==fa[x])return x;
	else return fa[x]=findfa(fa[x]);
}
void merge(int x,int y){
	int fx=findfa(x),fy=findfa(y);
	if(fx!=fy)fa[fx]=fy;
}
int minn=1,minx=0;
signed main(){
        std::ios::sync_with_stdio(0);
	//freopen("2024.in","r",stdin);
	std::cin>>n>>m;
	for(int i=1;i<=m;i++){
		int u,v;
		std::cin>>u>>v;
		g[u].emplace_back(v);
		g[v].emplace_back(u);
		++dgr[u];
		++dgr[v];
	}
	init();
	for(int i=2;i<=n;i++){
		if(dgr[i]<dgr[minn])minx=dgr[i],minn=i;
	}
	for(auto x:g[minn]){
		tag[x]=1;
	}
	for(int i=1;i<=n;i++){
		if(!tag[i]){
			fa[i]=minn;
			vis[i]=1;
		}
	}
	for(int i=1;i<=n;i++){
		if(!vis[i])s[++tot]=i;
	}
	memset(tag,0,sizeof tag);
	for(int i=1;i<=n;i++){
		int cnt=0;
		for(auto x:g[i]){
			tag[x]=1;
			cnt+=vis[x];
		}
		if(tot+cnt<n)merge(i,minn);
		for(int j=1;j<=tot;j++){
			if(!tag[s[j]])merge(i,s[j]);
			
		}
		for(auto x:g[i]){
			tag[x]=0;
		}
	}
	int ans=0;
	for(int i=1;i<=n;i++)ans+=(fa[i]==i);
	std::cout<<ans<<'\n';
	return 0;
}