///*** Sown_Vipro ***///
/// ->TEAM SELECTION TEST<- ///
#include<bits/stdc++.h>
using namespace std;
//#pragma GCC optimize ("O3")
//#pragma GCC optimize ("unroll-loops")
//#pragma GCC target("popcnt")
#define F first
#define S second
#define pb push_back
#define pi pair<int, int>
#define pii pair<int, pair<int, int> >
#define FOR(i, a, b) for(int i = a; i <= b; ++i)
#define REP(i, a, b) for(int i = a; i >= b; --i)
#define all(s) s.begin(), s.end()
#define szz(s) int(s.size())
const string NAME = "sown";
const int N = 1e6 + 5, MAX = 1e6, MOD = 1e9 + 7;
void maxi(int &x, int y){ if(x < y) x = y; }
void mini(int &x, int y){ if(x > y) x = y; };
void add(int &x, int y){ x += y; x += MOD * (x < 0); x -= MOD * (x >= MOD); };
int n, q, res, TIME, stt;
int s[N], c[N], nxt[N], ver[N], chain[N], p[N], in[N], icon[N], a[N], d[N];
vector<int> e[N];
void pre(int u){
s[u] = 1;
c[u] = (e[u].size() == 1);
for(int v : e[u]){
if(v == p[u]) continue;
p[v] = u;
pre(v);
s[u] += s[v];
if(s[v] > s[nxt[u]]) nxt[u] = v;
c[u] += c[v];
}
}
pi st[4 * N];
pi Merge(pi A, pi B){
return {A.F + B.F, A.S + B.S};
}
void build(int id, int l, int r){
if(l == r){
if(ver[l] != 1){
st[id].F = (c[ver[l]] % 2 == 0);
st[id].S = (c[ver[l]] % 2 != 0);
}
return;
}
int m = (l + r) / 2;
build(2 * id, l, m);
build(2 * id + 1, m + 1, r);
st[id] = Merge(st[2 * id], st[2 * id + 1]);
}
void update(int id, int l, int r, int u, int v){
if(l > v || r < u) return;
if(u <= l && r <= v){
swap(st[id].F, st[id].S);
return;
}
int m = (l + r) / 2;
update(2 * id, l, m, u, v);
update(2 * id + 1, m + 1, r, u, v);
st[id] = Merge(st[2 * id], st[2 * id + 1]);
}
void hld(int u, int p){
if(!icon[stt]) icon[stt] = u;
chain[u] = stt;
in[u] = ++TIME;
ver[TIME] = u;
if(nxt[u]) hld(nxt[u], u);
for(int v : e[u]){
if(v != p && v != nxt[u]){
++stt;
hld(v, u);
}
}
}
void solve(){
cin >> n >> q;
FOR(i, 2, n){
int u, v; cin >> u >> v;
e[u].pb(v);
e[v].pb(u);
}
pre(1);
hld(1, 0);
build(1, 1, n);
// cout << st[1].F;
while(q--){
int s; cin >> s;
int leaves = 0, S = 0;
FOR(i, 1, s){
cin >> a[i];
leaves += (e[a[i]].size() == 1);
S -= d[a[i]];
++d[a[i]];
S += d[a[i]];
}
if((c[1] + S - leaves) % 2){
cout << -1 << "\n";
continue;
}
FOR(i, 1, s){
int u = a[i];
while(chain[u] != chain[1]){
update(1, 1, n, in[icon[chain[u]]], in[u]);
cout << icon[chain[u]] << " " << u << "\n";
u = p[icon[chain[u]]];
}
update(1, 1, n, 1, in[u]);
}
cout << n + s - 1 + st[1].F << "\n";
FOR(i, 1, s){
int u = a[i];
while(chain[u] != chain[1]){
update(1, 1, n, in[icon[chain[u]]], in[u]);
u = p[icon[chain[u]]];
}
update(1, 1, n, 1, in[u]);
}
}
}
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(0); cout.tie(0);
if(fopen((NAME + ".inp").c_str(), "r")){
freopen((NAME + ".inp").c_str(), "r", stdin);
// freopen((NAME + ".out").c_str(), "w", stdout);
}
int t = 1;
// cin >> t;
while(t--){
solve();
}
}
