#include <cstdio>
#include <vector>

using namespace std;

FILE *fin, *fout;

#define NMAX 100000
vector<int> graph[NMAX + 5];
int n;
bool euler()
{
    int i;
    for(i = 1; i <= n; i++)
    {
        if(graph[i].size() % 2 != 0)
            return false;
    }

    return true;
}

int main()
{
    fin = fopen("ciclueuler.in", "r");
    fout = fopen("ciclueuler.out", "w");

    int m;
    fscanf(fin, "%d%d", &n, &m);

    int u, v, i;

    for(i = 1; i <= m; i++)
    {
        fscanf(fin, "%d%d", &u, &v);

        graph[u].push_back(i);
        graph[v].push_back(i);
    }

    /*for(i = 1; i <= n; i++)
    {
        fprintf(fout, "%d: ", i);
        for(auto it : graph[i])
        {
            fprintf(fout, "%d ", it);

        }
        fprintf(fout, "\n");
    }*/

    if(euler() == false)
        fprintf(fout, "-1");


    fclose(fin);
    fclose(fout);
    return 0;
}