Shared code

import java.io.PrintWriter;
import java.util.*;
 
public class MailDelivery {
    public static void main(String args[]) throws Exception {
 
        Scanner sc = new Scanner(System.in);
        PrintWriter pw = new PrintWriter(System.out);
        new MailDelivery().solve(sc, pw);
        sc.close();
        pw.flush();
        pw.close();
        System.exit(0);
    }
 
    void solve(Scanner sc, PrintWriter pw) throws Exception {
        int n = sc.nextInt();
        int m = sc.nextInt();
        ArrayDeque<Integer> adj[] = new ArrayDeque[n];
        for (int i = 0; i < n; i++) {
            adj[i] = new ArrayDeque<>();
        }
        for (int i = 0; i < m; i++) {
            int u = sc.nextInt() - 1;
            int v = sc.nextInt() - 1;
            adj[u].add(v);
            adj[v].add(u);
        }
        ArrayList<Integer> circuit = new ArrayList<>(n+1);
        int oddCount = 0;
        for (int i = 0; i < n; i++) {
            if(adj[i].size()%2 == 1){
                oddCount++;
            }
        }
        // System.out.println(oddCount);
        if(oddCount != 0 ){
            System.out.println("IMPOSSIBLE");
            return;
        }
        ArrayDeque<Integer> s = new ArrayDeque<>();
        s.push(0);
        int curr_v = 0;
        int g[][] = new int[n][n];
        boolean flag = false;
        while (!s.isEmpty()) {
 
            flag = false;
            while(adj[curr_v].size() > 0) {
                //System.out.println(curr_v);
                int next_v = adj[curr_v].pop();
                if(g[curr_v][next_v] == 1){
                    continue;
                }
                flag = true;
                s.push(curr_v);
                g[curr_v][next_v] = 1;
                g[next_v][curr_v] = 1;
                curr_v = next_v;
            }
            if(!flag){
                circuit.add(curr_v + 1);
                curr_v = s.pop();
            }
        }
 
        for (int i = 0; i < n; i++) {
            if(adj[i].size() > 0){
                System.out.println("IMPOSSIBLE");
            }
 
        }
 
        for (int i = 0; i < circuit.size(); i++) {
            if (i == 0) {
                System.out.print(circuit.get(i));
            } else {
                System.out.print(" " + circuit.get(i));
            }
 
        }
        System.out.println();
 
    }
}