#include <stdio.h>
#define nmax 100

struct nod
{
int x;
nod *urm;
};

nod *A[nmax];
int grad[nmax],st[nmax],k=0;

void del(int x,int y)
{
if (A[x]->x==y) A[x] = A[x]->urm;
else
{
while (A[x]->urm->x!=y && A[x]->urm!=NULL) A[x]=A[x]->urm;
if (A[x]->urm==NULL) return;
A[x]->urm=A[x]->urm->urm;
}
}

void add(int x,int y)
{
nod *urm;
urm = new nod;
urm->x= y;
urm->urm = A[x];
A[x] = urm;
}

int main()
{
freopen("ciclueuler.in","r",stdin);
freopen("ciclueuler.out","w",stdout);

int n,m,x,y,i;

scanf("%d%d\n",&n,&m);
while (m)
{
m--;
scanf("%d%d\n",&x,&y);
grad[x]++;
grad[y]++;
if (x==y) add(x,y);
else add(x,y),add(y,x);
}


for (i=1;i<=n;i++) if (grad[i]%2==1)
				  {
				   printf("-1");
				   return 0;
				  };

//nod *w;
//k=1;
//st[k]=1;


//while (k)
//{
//x = st[k];
//if (A[x]!=NULL) {
//		st[++k] = A[x]->x;
//		if (A[x]->x!=x) del(A[x]->x,x);
//		//w = A[x];
//		A[x]=A[x]->urm;
//		//delete w;
//		}
//else if (k>1) printf("%d ",st[k--]);
//     else k--;
//}
return 0;
}