#include <bits/stdc++.h>

using namespace std;

#define rep(i, s, e) for (int i = s; i <= e; ++i)
#define drep(i, s, e) for (int i = s; i >= e; --i)
#define file(a) freopen(#a".in", "r", stdin), freopen(#a".out", "w", stdout)
#define pv(a) cout << #a << " = " << a << endl
#define pa(a, l, r) cout << #a " : "; rep(_, l, r) cout << a[_] << ' '; cout << endl

const int N = 1e5 + 10;

int read() {
  int x = 0, f = 1; char c = getchar();
  for (; c < '0' || c > '9'; c = getchar()) if (c == '-') f = -1;
  for (; c >= '0' && c <= '9'; c = getchar()) x = x * 10 + c - 48;
  return x * f;
}

int n, m, x[N], y[N];
vector <pair <int, int>> e[N];

int fa[N];
int get(int u) { return u == fa[u] ? u : fa[u] = get(fa[u]); }
bool merge(int u, int v) {
  if (get(u) == get(v)) return false;
  fa[get(v)] = get(u);
  return true;
}

int st[N], top, cur[N];

void dfs(int u, int fa, int t) {
  if (u == t) {
    rep(i, 1, top) cur[i] = st[i];
    sort(cur + 1, cur + top + 1);
    rep(i, 1, top) printf("%d ", cur[i]);
    return;
  }
  for (auto [v, i] : e[u]) if (v != fa) {
    st[++ top] = i, dfs(v, u, t), -- top;
  }
}

void solve() {
  n = read(), m = read();
  rep(i, 1, n) e[i].clear(), fa[i] = i;
  rep(i, 1, m) x[i] = read(), y[i] = read();
  rep(i, 1, m) {
    if (merge(x[i], y[i])) {
      e[x[i]].emplace_back(make_pair(y[i], i));
      e[y[i]].emplace_back(make_pair(x[i], i));
      continue;
    }
    top = 0, dfs(x[i], 0, y[i]);
    printf("%d\n", i);
    return;
  }
  printf("-1\n");
}

int main() {
  for (int tc = read(); tc; -- tc) solve();
  return 0;
}