/* Problem URL: https://cses.fi/problemset/task/1691 */

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

using namespace std;
using namespace __gnu_pbds;

template <class T, class comp = less<>>
using ordered_set = tree<T, null_type , comp , rb_tree_tag , tree_order_statistics_node_update>;

#define V vector

#define rmin(a, b) a = min(a, b)
#define rmax(a, b) a = max(a, b)

#define rep(i, lim) for (int i = 0; i < (lim); i++)
#define nrep(i, s, lim) for (int i = s; i < (lim); i++)

#define repv(i, v) for (auto &i : (v))
#define fillv(v) for (auto &itr_ : (v)) { cin >> itr_; }
#define sortv(v) sort(v.begin(), v.end())
#define all(v) (v).begin(), (v).end()

using vi = vector<int>;
using vvi = vector<vi>;
using vvvi = vector<vvi>;
using vvvvi = vector<vvvi>;

using ll = long long;

using vl = vector<ll>;
using vvl = vector<vl>;
using vvvl = vector<vvl>;
using vvvvl = vector<vvvl>;

template<class v>
auto operator<<(ostream &os, const vector<v> &vec)->ostream& {
    os << vec[0];
    for (size_t i = 1; i < vec.size(); i++) {
        os << ' ' << vec[i];
    }
    os << '\n';
    return os;
}

template<class v>
auto operator>>(istream &is, vector<v> &vec)->istream& {
    for (auto &i : vec) {
        is >> i;
    }
    return is;
}

template<class v>
auto operator<<(ostream &os, const vector<vector<v>> &vec)->ostream& {
    for (auto &i : vec) {
        os << i[0];
        for (size_t j = 1; j < i.size(); j++) {
            os << ' ' << i[j];
        }
        os << '\n';
    }
    return os;
}

template<class v>
auto operator>>(istream  &is, vector<vector<v>> &vec)->istream& {
    for (auto &i : vec) {
        for (auto &j : i) {
            is >> j;
        }
    }
    return is;
}

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int n, m;
    cin >> n >> m;

    V<V<pair<int, int>>> graph(n);
    V<bool> used(m);
    rep(i, m) {
        int a, b;
        cin >> a >> b;
        a--, b--;

        graph[a].emplace_back(b, i);
        graph[b].emplace_back(a, i);
    }

    rep(i, n) {
        if (graph[i].size() & 1) {
            cout << "IMPOSSIBLE\n";
            return 0;
        }
    }

    vi ans;
    stack<int> euler;
    euler.push(0);
    while (!euler.empty()) {
        int i = euler.top();
        while (!graph[i].empty() && used[graph[i].back().second]) {
            graph[i].pop_back();
        }

        if (graph[i].empty()) {
            ans.push_back(i + 1);
            euler.pop();
            continue;
        }

        used[graph[i].back().second] = true;
        euler.push(graph[i].back().first);
        graph[i].pop_back();
    }

    if (ans.size() != m + 1) {
        cout << "IMPOSSIBLE\n";
        return 0;
    }

    cout << ans;
}