#include <fstream>
#include <vector>
#include <bitset>

#define Nmax 100001

using namespace std;

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

int n, m, x, y;
vector < int > G[Nmax];

int main()
{
    fin >> n >> m;
    while (fin >> x >> y)
    {
        G[x].push_back(y);
        G[y].push_back(x);
    }

    for (i = 1; i <= n; i ++)
    {
        if (G[i].size() % 2)
            fout << -1;
    }

    fin.close();
    fout.close();
    return 0;
}