#include<iostream>
#include<list>
#include<queue>
#include<algorithm>
using namespace std;
int n,m;
list<int> a[100001];
list<int> nodes;
queue<int> q;
int main(){
	freopen("dfs.in", "r", stdin);
	freopen("dfs.out", "w", stdout);
	int n1,n2;
    scanf("%d %d",&n,&m);
    for(int i=1;i<=n;i++){
    	nodes.push_back(i);
    }
    for(int i=1;i<=m;i++){
    	scanf("%d %d",&n1,&n2);
    	a[n1].push_back(n2);
    	a[n2].push_back(n1);
    }
    q.push(1);
    int c=0;
    while(q.size()!=0){
    	int p=q.front();
    	q.pop();
    	if(find(nodes.begin(), nodes.end(), p) != nodes.end()){
    		nodes.remove(p);
	    	for(list<int>::iterator it=a[p].begin();it!=a[p].end();){
	    		q.push(*it);
	    		a[p].erase(it++);
	    	}
    	}
    	if(q.size()==0){
    		c++;
    		if(nodes.size()!=0)q.push(*next(nodes.begin(),0));
    	}
    }
    printf("%d",c);
}