并查集维护相邻的点不可再选，按边的数量排序，从最多出边的点开始找，贪心思维，找完之后如果没有填满所有边，就输出不可能

#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;
const int N = 1e6;
int n,m;
int ans = 0;
struct edge{
	vector<int> v;
	int point;
};
edge a[N];
int f[N];
void init(){
	for(int i = 1;i<=n;++i) f[i] = i;
}
int findfather(int x){
	if(x==f[x]) return x;
	return f[x] = findfather(f[x]);
}
void merge(int a,int b){
	int fa = findfather(a);
	int fb = findfather(b);
	if(fa!=fb){
		f[fa] = fb;
	}
}
bool cmp(edge a,edge b){
	return a.v.size()>b.v.size();
}
int main(){
	cin>>n>>m;
	init();
	for(int i = 1;i<=m;++i){
		int x,y;
		cin>>x>>y;
		a[x].point = x;
		a[x].v.push_back(y);		
		a[y].point = y;
		a[y].v.push_back(x);
	}
	sort(a+1,a+n+1,cmp);
	int t = 0;
	for(int i = 1;i<=n;++i){
		if(findfather(a[i].point) == a[i].point){
			for(int j = 0;j<a[i].v.size();++j){
				merge(a[i].v[j],a[i].point);
			}
			ans+=a[i].v.size();
			t++;
		}
	}
	if(ans<m) cout<<"Impossible";
	else cout<<t;
	return 0;
}