#include<stdio.h>
#define SIZE 100

int map[SIZE][SIZE];
int queue[SIZE * SIZE];
int color[SIZE];
int visit[SIZE];

int ncases;
int n,m;
int ans;

int head = 0;
int rear = 0;
int tab;
void bfs(int step)
{
    queue[rear] = step;
    rear++;
    color[step]=1;
    visit[step]=1;
    while(head < rear)
    {
        int temp = queue[head];
        head++;
        for(int i=1;i<=n;i++)
        {
            tab=0;
            if(map[temp][i] == 1)
            {
                if(color[i] == 0)
                {
                    color[i] -= color[temp];
                    queue[rear]=i;
                    rear++;
                    visit[i]=1;
                }
                else if(color[i] == color[temp])
                {
                    tab=1;
                    ans=-1;
                    break;
                }
            }
        }
        if(tab) break;
    }
}
int main(){
    freopen("input.txt","r",stdin);
    scanf("%d",&ncases);
    for(int i=0;i<ncases;i++)
    {
        scanf("%d%d",&n,&m);
        ans=0;
        int x,y;
        for(int i=1;i<=m;i++)
        {
            scanf("%d%d",&x,&y);
            map[x][y]=1;
            map[y][x]=1;
        }
        for(int i=1;i<=n;i++)
        {
            if(visit[i] == 0)
                bfs(i);
            if(tab) break;
        }
        if(ans==0)
        {
            for(int i=1;i<=n;i++)
            {
                if(color[i] == -1)
                    ans++;
            }
            printf("%d\n",ans);
            for(int i=1;i<=n;i++)
            {
                if(color[i] == -1)
                    printf("%d ",i);
            }
            printf("\n");
        }
        else printf("%d\n",ans);
    }
}

/*测试用例 
9
5 5
1 2 2 3 3 4 4 5 5 1
6 6
1 2 2 3 3 4 4 5 5 6 6 1
4 5
1 2 2 3 3 4 4 1 4 2
4 6
1 2 2 3 3 4 4 1 4 2 1 3
4 4
1 2 2 3 3 4 4 1
5 6
1 2 1 4 2 3 2 5 3 4 5 4 
12 13
1 2 1 4 2 3 3 4 4 5 5 6 6 7 7 8 8 5 5 9 4 10 9 10 11 12
5 4
1 4 1 3 3 5 2 5
7 5
1 4 1 3 3 5 2 5 6 7
测试结果
-1
3
2 4 6 
-1
-1
2
2 4 
2
2 4 
6
2 4 6 8 9 12 
3
2 3 4 
4
2 3 4 7 */