#include <cassert>
#include <cmath>
#include <cstdint>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <functional>
#include <iostream>
#include <limits>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <sstream>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

using namespace std;

using Int = long long;

template <class T1, class T2> ostream &operator<<(ostream &os, const pair<T1, T2> &a) { return os << "(" << a.first << ", " << a.second << ")"; };
template <class T> ostream &operator<<(ostream &os, const vector<T> &as) { const int sz = as.size(); os << "["; for (int i = 0; i < sz; ++i) { if (i >= 256) { os << ", ..."; break; } if (i > 0) { os << ", "; } os << as[i]; } return os << "]"; }
template <class T> void pv(T a, T b) { for (T i = a; i != b; ++i) cerr << *i << " "; cerr << endl; }
template <class T> bool chmin(T &t, const T &f) { if (t > f) { t = f; return true; } return false; }
template <class T> bool chmax(T &t, const T &f) { if (t < f) { t = f; return true; } return false; }
#define COLOR(s) ("\x1b[" s "m")


int N;
vector<int> A, B;

vector<vector<int>> graph;
vector<int> par;

/*
  dp[u][k]: k paths waiting at u
  up
    0 -> 1
    1 -> 0,1
*/
vector<int> dp;
void dfs(int u, int p) {
  par[u] = p;
  int cnt[2] = {};
  for (const int v : graph[u]) if (p != v) {
    dfs(v, u);
    ++cnt[dp[v] >> 1 & 1];
  }
  if (cnt[1]) {
    dp[u] = 1 << 0 | 1 << 1;
  } else {
    dp[u] = 1 << (cnt[0] & 1);
  }
}

vector<pair<int, int>> ans;
int DFS(int u, int k) {
  assert(dp[u] >> k & 1);
  vector<int> vss[2];
  for (const int v : graph[u]) if (par[u] != v) {
    vss[dp[v] >> 1 & 1].push_back(v);
  }
  // match 0
  int ret = -1;
  for (const int v : vss[0]) {
    DFS(v, 0);
    if (~ret) {
      ans.emplace_back(ret, v);
      ret = -1;
    } else {
      ret = v;
    }
  }
  if (vss[1].size()) {
    for (int j = (int)vss[1].size(); --j >= 0; ) {
      const int v = vss[1][j];
      const int res = DFS(v, 1);
      if (j) {
        // docchidemo
        if (~ret) {
          ans.emplace_back(ret, res);
          ret = -1;
        } else {
          ret = res;
        }
      } else {
        if (k) {
          if (~ret) {
            ans.emplace_back(u, res);
          } else {
            ret = res;
          }
        } else {
          if (~ret) {
            ans.emplace_back(ret, res);
            ret = -1;
          } else {
            ans.emplace_back(u, res);
          }
        }
      }
    }
  }
// cerr<<"[DFS] "<<u<<" "<<k<<"; ans = "<<ans<<", ret = "<<ret<<endl;
  if (k) {
    assert(~ret);
  } else {
    assert(!~ret);
  }
  return ret;
}

int main() {
  for (int numCases; ~scanf("%d", &numCases); ) { for (int caseId = 1; caseId <= numCases; ++caseId) {
    scanf("%d", &N);
    A.resize(N - 1);
    B.resize(N - 1);
    for (int i = 0; i < N - 1; ++i) {
      scanf("%d%d", &A[i], &B[i]);
      --A[i];
      --B[i];
    }
    
    graph.assign(N, {});
    for (int i = 0; i < N - 1; ++i) {
      graph[A[i]].push_back(B[i]);
      graph[B[i]].push_back(A[i]);
    }
    par.assign(N, -1);
    dp.assign(N, 0);
    dfs(0, -1);
// cerr<<"dp = "<<dp<<endl;
    
    if (dp[0] >> 0 & 1) {
      ans.clear();
      DFS(0, 0);
      printf("%d\n", (int)ans.size());
      for (const auto &op : ans) {
        printf("%d %d\n", op.first + 1, op.second + 1);
      }
    } else {
      puts("-1");
    }
  }
#ifndef LOCAL
  break;
#endif
  }
  return 0;
}