#include<bits/stdc++.h>
using namespace std;
const int N=100005;
int n,m,k,s,t,a[N][2],bo,vis[N],vis1[N];
vector<pair<int,int>> e[N];
int b[N],tot,c[N],tot1;
void dfs(int u,int ID){
	vis[u]=1;
	for (auto [v,id]:e[u]){
		if (id==ID) continue;
		if (vis[v]){
			if (!bo) {bo=a[id][0];c[++tot1]=id;}
			continue;
		}
		dfs(v,id);
	}
}
void dfs1(int u,int ID){
	vis1[u]=1;
	for (auto [v,id]:e[u]){
		if (id==ID || vis1[v]) continue;
		b[++tot]=id;
		dfs1(v,id);
	}
}
int main(){
    cin>>n>>m;
    for (int i=1;i<=n;++i){
		cin>>s>>t;
		a[i][0]=s;a[i][1]=t;
		e[s].push_back({t,i});
		e[t].push_back({s,i});
	}
	for (int i=1;i<=m;++i)
		if (!vis[i]){
			bo=0;	
			dfs(i,0);
			dfs1(bo?bo:i,c[tot1]);
		}
	cout<<n-tot1-tot<<"\n";
	memset(vis,0,sizeof(vis));
	for (int i=1;i<=tot1;++i) cout<<c[i]<<"\n",vis[c[i]]=1;
	for (int i=1;i<=tot;++i) cout<<b[i]<<"\n",vis[b[i]]=1;
	for (int i=1;i<=n;++i) if (!vis[i]) cout<<i<<"\n";
}