#include <cstdio>
#include <vector>
#include <queue>
#define nmax 100009
using namespace std;
struct vecini
{
    vector<int> v;
};
vecini g[nmax];
int deg[nmax];
vector <int> a1;vector <int>a2;
int suma;
queue<int> q;
bool viz[nmax];
int main()
{
    freopen("ciclueuler.in","r",stdin);
    freopen("ciclueuler.out","w",stdout);
    int i,j,n,m,x,y;
    scanf("%d%d",&n,&m);
    suma=2*m;
    for(i=1;i<=n;i++)
    {
        scanf("%d%d",&x,&y);
        g[x].v.push_back(y);deg[x]++;
        if(x!=y)
        {
            g[y].v.push_back(x); deg[y]++;
        }
    }
    q.push(1);
    viz[1]=true;
    while(q.empty()==0)
    {
        int top=q.front();
        int lung=g[top].v.size();
        for(i=0;i<lung;++i)
        {
            if(viz[g[top].v[i]]==false)
            {
                viz[g[top].v[i]]=true;
                q.push(g[top].v[i]);
            }
        }
        q.pop();
    }
    for(i=1;i<=n;i++)
    {
        if(viz[i]==false||deg[i]%2!=0)
        {
            printf("-1\n");
            return 0;
        }
    }
    /*while(suma!=0)
    {
        for(i=1;i<=n;i++)
            if(deg[i]!=0)
            {
                DFS(i);
                break;
            }
        lung=a2.size();
        for(i=0;i<lung;++i)
        {
            if(deg[a2[i]]==0)
                printf("%d",a2[i]);
            else
                for(j=i;j<=lung;j++)
                {
                    a1.push_back(j);
                }
        }
    }
    return 0;*/
}