#include <fstream>
#include <vector>
#include <cstdlib>
#include <list>

std::ifstream be ("ciclueuler.in");
std::ofstream ki ("ciclueuler.out");

int n, m;
std::vector< std::list<int> > x;

void fokszam()
{
    for (int i=1; i<=n; i++)
        if (x[i].size() % 2 || x[i].size() == 0) {
            ki << -1;
            exit(0);
        }
}

int main()
{
    be >> n >> m;
    x.resize (n+1);

    int a, b;
    for (int i=0; i<m; i++) {
        be >> a >> b;
        x[a].push_back (b);
        x[b].push_back (a);
    }

    fokszam();
}