#include <bits/stdc++.h>

using namespace std;

// trip to new record: 2100 hard (last record: 1800)

vector<vector<pair<int, int>>> g;
vector<bool> used;
vector<int> tin;
vector<int> tup;
int timer = 0;
vector<int> ans;

void dfs1(int v, int pr, int ind) { // dfs tree
    used[v] = 1;
    tin[v] = timer++;
    tup[v] = tin[v];
    for(int i = 0; i < g[v].size(); i++) {
        int to = g[v][i].first;
        if(g[v][i].second == ind) continue;
        if(!used[to]) {
            dfs1(to, v, g[v][i].second);
            tup[v] = min(tup[v], tup[to]);
        }
        else {
            tup[v] = min(tup[v], tin[to]);
        }
    }
    if(tin[pr] < tup[v]) {
        ans.push_back(ind);
    }
}

vector<bool> used3;
int mx_ans = 0;
int versh = 0;
void dfs2(int v, int depth) { // 1 case
    if(depth > mx_ans) {
        mx_ans = depth;
        versh = v;
    }
    used3[v] = true;
    for(int i = 0; i < g[v].size(); i++) {
        int to = g[v][i].first;
        if(!used3[to]) {
            dfs2(to, ++depth);
        }
    }
}

vector<int> pr;
vector<bool> used2;
void dfs3(int v, int pred) { // 2 case
    used2[v] = true;
    for(int i = 0; i < g[v].size(); i++) {
        int to = g[v][i].first;
        if(to == pred) continue;
        if(!used2[to] && tup[to] == 0) {
            pr[to] = v;
            dfs3(to, v);
        }
    }
    int ver = v;
    vector<int> path;
    while(ver != 0) {
        path.push_back(ver);
        ver = pr[ver];
    }
    path.push_back(0);
    reverse(path.begin(), path.end());
    for(auto& i : path) i++;
    cout << path.size() << '\n';
    for(int i = 0; i < path.size(); i++) cout << path[i] << ' ';
    exit(0);
}

int main() {
    int n, m;
    cin >> n >> m;
    g.resize(n);
    tin.resize(n);
    tup.resize(n);
    used.resize(n);
    pr.resize(n);
    used2.resize(n);
    used3.resize(n);
    for(int i = 0; i < m; i++) {
        int a, b;
        cin >> a >> b;
        if(a > b) {
            swap(a, b);
        }
        a--;
        b--;
        g[a].push_back({b, i});
        g[b].push_back({a, i});
    }
    for(int i = 0; i < n; i++) {
        if(!used[i]) {
            dfs1(i, i, -1);
        }
    }

    if(count(tup.begin(), tup.end(), 0) == 1) {
        cout << -1 << '\n';
        dfs2(0, 0);
        cout << versh + 1;
    }
    else {
        dfs3(0, 0);
    }
}