#include <bits/stdc++.h>

using namespace std;
#define IOS ios::sync_with_stdio(false);cin.tie(nullptr);cout.tie(nullptr)
#define rep(a, b, c) for(int (a)=(b);(a)<=(c);(a)++)
#define per(a, b, c) for(int (a)=(b);(a)>=(c);(a)--)
#define mset(var, val) memset(var,val,sizeof(var))
#define ll long long
#define int ll
#define fi first
#define se second
#define no "NO\n"
#define yes "YES\n"
#define pb push_back
#define endl "\n"
#define pii pair<int,int>
#define pll pair<ll,ll>
#define dbg(x...) do{cout<<#x<<" -> ";err(x);}while (0)

void err() { cout << '\n'; }

template<class T, class... Ts>
void err(T arg, Ts... args) {
    cout << arg << ' ';
    err(args...);
}

const int N = 2e5 + 5;
const int M = 1e6 + 5;
const int inf = 0x3f3f3f3f;
const ll INF = 0x3f3f3f3f3f3f3f3f;
const int P = 1e9 + 7;
const double eps = 1e-8;
const double pi = acos(-1.0);

int n, m, s, t, sqn, cnt, dep[N];
int tot=0, lnk[N], son[M<<1], nxt[M<<1];
bool qaq[N], f[N];

queue<int>q;

void add(int x, int y) {
    son[++tot] = y, nxt[tot] = lnk[x], lnk[x] = tot;
}

void bfs1(int x) {
    while(!q.empty()) q.pop();

    q.push(x); dep[x] = 1;
    while(!q.empty()) {
        int u = q.front(); q.pop();
        for(int i = lnk[u]; i != -1; i = nxt[i]) {
            int v = son[i];
            if(dep[v]) continue;
            dep[v] = dep[u]+1;
            q.push(v);
        }
    }
}

bool check(int x) {
    if(x <= sqn) return 1;
    int xx = (x-sqn) / (2*sqn);
    xx = xx*2*sqn + sqn;
    if(x-xx > sqn && x-xx < 2*sqn) return 1;
    return 0;
}

void bfs2(int x) {
    while(!q.empty()) q.pop();
    q.push(x); dep[x] = 1;
    while(!q.empty()) {
        int u = q.front(), out=0; q.pop();
        for(int i = lnk[u]; i != -1; i = nxt[i]) {
            int v = son[i];
            if(dep[v]) continue;
            dep[v] = dep[u]+1, out++;
            if(qaq[dep[v]]) cnt++, f[v] = 1;
            q.push(v);
        }
        if(!out && check(dep[u])) cnt++, f[u] = 1;
    }
}

void solve() {
    cin >> n >> m;
    sqn = sqrt(n-1)+1, tot=0;
    for(int i = 1; i <= n; i++) {
        lnk[i] = -1, dep[i] = 0, f[i] = 0;
        if(i%sqn == 0 && ((i/sqn)&1)) qaq[i] = 1;
        else qaq[i] = 0;
    }
    for(int i = 1; i <= m; i++) {
    	int x, y;
        cin >> x >> y;
        add(x, y); add(y, x);
    }

    bfs1(1); s = 1;
    for(int i = 1; i <= n; i++)
        if(dep[i] > dep[s]) s = i;
    for(int i = 1; i <= n; i++) dep[i] = 0;
    // printf("s%d\n", s);

    bfs1(s); t = 1;
    for(int i = 1; i <= n; i++)
        if(dep[i] > dep[t]) t = i;
    for(int i = 1; i <= n; i++) dep[i] = 0;
    // printf("t%d\n", t);

    cnt=0;
    bfs2(t);

    if(cnt <= sqn) {
        printf("%d\n", cnt);
        for(int i = 1; i <= n; i++)
            if(f[i]) printf("%d ", i);
        printf("\n");
    }
    else printf("-1\n");
}

signed main() {
    IOS;
    int T = 1;
    cin >> T;
    while (T--) {
        solve();
    }
    return 0;
}