#include <stdio.h>
#define MaxN 100001
int n, m;
struct nod {
	int inf, edge;
	nod *urm;
};
nod *L[MaxN];
bool viz[MaxN];
int grad[MaxN];
void add(int ex1, int ex2){
	nod *p, *q;
	p = new nod;
	p->inf = ex2;
	p->urm = L[ex1];
	L[ex1] = p;
	q = new nod;
	q->inf = ex1;
	q->urm = L[ex2];
	L[ex2] = q;
};

void DFS(int x){
	viz[x] = true;
	
	for (nod *p = L[x]; p != NULL; p = p->urm)
		if (!viz[p->inf]){
			p->edge = 1;
			for (nod *q = L[p->inf]; q != NULL && q->inf != x; q = q->urm)
				if (q->inf == x) q->edge = 1;
			DFS(p->inf);
		};
};

int main(){
	int ex1, ex2;
	freopen("ciclueuler.in","r",stdin);
	freopen("ciclueuler.out","w",stdout);
	scanf("%d %d",&n, &m);
	for (int i = 1; i <= m; i++){
		scanf("%d %d", &ex1, &ex2);
		add(ex1,ex2);
		grad[ex1]++;
		grad[ex2]++;
	};
	DFS(1);
	for (int i = 1; i <= n; i++){
		if (grad[i] % 2) {
			printf("-1");
			return 0;
		};
		if (viz[ i ] != true )  {
			printf("-1");
			return 0;
		};
	};
};