#include <fstream>

using namespace std;
ifstream f("ciclueuler.in");
ofstream g("ciclueuler.out");
struct nod
{
    int val;
    nod *urm;
};
typedef nod *pnod;
pnod v[100003],p;
int n,m,t[100003];
void ad(int x,int y)
{
    p=new nod;
    p->urm=v[x]->urm;
    p->val=y;
    v[x]->urm=p;
    v[x]->val+=1;
}
bool bun()
{
    int i;
    for(i=1;i<=n;i++)
        if(v[i]->val%2==1)
            return 0;
    return 1;
}
bool viz[100003];
bool conex()
{
    t[1]=1;
    int k=1,s=1;
    viz[1]=1;
    while(k<=s)
    {
        p=v[t[k]]->urm;
        while(p)
        {
            if(viz[p->val]==0)
            {
                viz[p->val]=1;
                s+=1;
                t[s]=p->val;
            }
            p=p->urm;
        }
        k+=1;
    }
    if(s==n)
        return 1;
    return 0;
}
int main()
{
    int i,x,y;
    f>>n>>m;
    for(i=1;i<=n;i++)
    {
        v[i]=new nod;
        v[i]->val=0;
        v[i]->urm=0;
    }
    for(i=1;i<=m;i++)
    {
        f>>x>>y;
        ad(x,y);
        ad(y,x);
    }
    if(bun()==0)
        g<<"-1";
    if(conex()==0)
        g<<"-1";
    return 0;
}