#include <array>
#include <cmath>
#include <string>
#include <vector>
#include <cstdint>
#include <iostream>
#include <algorithm>
using namespace std;

uint64_t seed = 1234567891234567891;
uint64_t xorshift64() {
	seed ^= seed << 13;
	seed ^= seed >> 17;
	seed ^= seed << 7;
	return seed;
}
int rand_int(int l, int r) {
	return l + int(xorshift64() % (r - l));
}

const int INF = 1012345678;

const vector<string> code_recov = { "AA", "AB", "BB", "AC", "BC", "CC", "AD", "BD", "CD", "DD" };

int get_code(char c1, char c2) {
	if (c1 > c2) {
		swap(c1, c2);
	}
	int x = c1 - 'A', y = c2 - 'A';
	return y * (y + 1) / 2 + x;
}

string subsolve(int N, const string& S, const string& T) {
	int M = T.size();
	array<int, 4> d1 = {0};
	array<int, 4> d2 = {0};
	for (int i = 0; i < N; i++) {
		d1[S[i] - 'A']++;
	}
	for (int i = 0; i < M; i++) {
		d2[T[i] - 'A']++;
	}
	string alpha;
	for (int i = 0; i < 4; i++) {
		if (d1[i] < d2[i]) {
			return string();
		}
		alpha += string(d1[i] - d2[i], 'A' + i);
	}
	array<int, 10> c1 = {0};
	array<int, 10> c2 = {0};
	for (int i = 0; i < N - 1;  i++) {
		c1[get_code(S[i], S[i + 1])]++;
	}
	for (int i = 0; i < M - 1; i++) {
		c2[get_code(T[i], T[i + 1])]++;
	}
	const int MAX_LOOPS = 500000;
	auto get_score = [&](const vector<int>& seq) -> int {
		array<int, 10> c = c2;
		for (int i = 0; i < N - M; i++) {
			if (c[seq[i]] == 0) {
				return INF;
			}
			c[seq[i]]--;
			c[get_code(alpha[i], code_recov[seq[i]][0])]++;
			c[get_code(alpha[i], code_recov[seq[i]][1])]++;
		}
		int score = 0;
		for (int i = 0; i < 10; i++) {
			score += abs(c[i] - c1[i]);
		}
		return score;
	};
	vector<int> curseq(N - M);
	for (int i = 0; i < N - M; i++) {
		curseq[i] = get_code(T[0], i != 0 ? alpha[i - 1] : T[1]);
	}
	int curscore = get_score(curseq);
	if (N > M) {
		int loops = 0;
		while (loops < MAX_LOOPS && curscore > 0) {
			loops++;
			vector<int> nxtseq = curseq;
			nxtseq[rand_int(0, N - M)] = rand_int(0, 10);
			int nxtscore = get_score(nxtseq);
			if (nxtscore != INF) {
				curscore = nxtscore;
				curseq = nxtseq;
			}
		}
	}
	if (curscore != 0) {
		return string();
	}
	string ans = T;
	for (int i = 0; i < N - M; i++) {
		for (int j = 0; j < int(ans.size()) - 1; j++) {
			if (get_code(ans[j], ans[j + 1]) == curseq[i]) {
				ans.insert(ans.begin() + j + 1, alpha[i]);
				break;
			}
		}
	}
	return ans;
};

string increase(string S, char head, char tail) {
	if (S[0] != head) S = head + S;
	if (S.back() != tail) S = S + tail;
	return S;
}

string solve(int N, int M, const string& S, const string& T) {
	string res1 = subsolve(N, S, increase(T, S[0], S.back()));
	if (!res1.empty()) {
		return res1;
	}
	if (S[0] != S[1]) {
		string res2 = subsolve(N, S, increase(T, S[1], S.back()));
		if (!res2.empty()) {
			return res2;
		}
	}
	return string();
}

int main() {
	int Q;
	cin >> Q;
	for (int id = 1; id <= Q; id++) {
		int N, M; string S, T;
		cin >> N >> M >> S >> T;
		string ans = solve(N, M, S, T);
		if (!ans.empty()) {
			cout << ans << endl;
		} else {
			cout << "-1" << endl;
		}
	}
	return 0;
}