#include <iostream>
#include <fstream>
#include <vector>

using namespace std;

ifstream fin("ciclueuler.in");
ofstream fout("ciclueuler.out");

int main() {
    int numar_noduri, numar_muchii;
    fin >> numar_noduri >> numar_muchii;
    vector<vector<int>> lista_vecini(numar_noduri + 1);

    for (int i = 0; i < numar_muchii; i++) {
        int nod1, nod2;
        fin >> nod1 >> nod2;
        lista_vecini[nod1].push_back(nod2);
        lista_vecini[nod2].push_back(nod1);
    }

    for (int i = 1; i <= numar_noduri; i++) {
        if (lista_vecini[i].size() % 2 == 1) {
            fout << "-1" << endl;
            return 0;
        }
    }



    return 0;
}