///EULERIAN CYCLE HOME 04.01
#include <iostream>
#include <fstream>
#include <vector>
#include <list>
#include <stack>

using namespace std;

list<int> cycle;
vector<list<int> > adjList;

void itEuler(vector<list<int> >& adjList, list<int>& cycle) {
    stack<int> s;
    s.push(0);

    int current, next;
    list<int>::iterator it;
    while(!s.empty()) {
        current = s.top();

        if(adjList[current].empty()) {
            s.pop();
            cycle.push_back(current);

            continue;
        }

        next = adjList[current].front();
        adjList[current].pop_front();

        s.push(next);

        for(it = adjList[next].begin(); it != adjList[next].end(); it++)
            if(*it == current) {
                adjList[next].erase(it);
                break;
            }
    }
}

void recEuler(int v) {
    while(!adjList[v].empty()) {
        int w = adjList[v].front();
        adjList[v].pop_front();
        for(list<int>::iterator it = adjList[w].begin(); it != adjList[w].end(); it++)
            if(*it == v) {
                adjList[w].erase(it);
                break;
            }

        recEuler(w);
    }
    cycle.push_back(v);
}

int main() {
    ifstream inStr("ciclueuler.in");
    ofstream outStr("ciclueuler.out");

    int N, M;

    inStr >> N >> M;

    adjList.resize(N);
    vector<int> numEdges(N, 0);

    int from, to;
    for(int i=0; i<M; i++) {
        inStr >> from >> to;
        adjList[--from].push_back(--to);
        adjList[to].push_back(from);
        ++numEdges[from];
        ++numEdges[to];
    }

    for(int i=0; i<N; i++)
        if(numEdges[i] % 2 != 0) {
            cout << -1 << '\n';
            return 0;
        }

    ///recEuler(0);
    ///itEuler(adjList, cycle);

    ///for(list<int>::iterator it = cycle.begin();
            ///it != cycle.end(); it++)
        ///outStr << *it + 1 << ' ';
    outStr << '\n';

    return 0;
}