#include "bits/stdc++.h"
// #pragma GCC optimize("O3,unroll-loops")
// #pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")
using namespace std;
using ll = long long int;
mt19937_64 rng(chrono::high_resolution_clock::now().time_since_epoch().count());

struct input_checker {
	string buffer;
	int pos;

	const string all = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
	const string number = "0123456789";
	const string upper = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
	const string lower = "abcdefghijklmnopqrstuvwxyz";

	input_checker() {
		pos = 0;
		while (true) {
			int c = cin.get();
			if (c == -1) {
				break;
			}
			buffer.push_back((char) c);
		}
	}

	int nextDelimiter() {
		int now = pos;
		while (now < (int) buffer.size() && buffer[now] != ' ' && buffer[now] != '\n') {
			now++;
		}
		return now;
	}

	string readOne() {
		assert(pos < (int) buffer.size());
		int nxt = nextDelimiter();
		string res;
		while (pos < nxt) {
			res += buffer[pos];
			pos++;
		}
		// cerr << res << endl;
		return res;
	}

	string readString(int minl, int maxl, const string &pattern = "") {
		assert(minl <= maxl);
		string res = readOne();
		assert(minl <= (int) res.size());
		assert((int) res.size() <= maxl);
		for (int i = 0; i < (int) res.size(); i++) {
			assert(pattern.empty() || pattern.find(res[i]) != string::npos);
		}
		return res;
	}

	int readInt(int minv, int maxv) {
		assert(minv <= maxv);
		int res = stoi(readOne());
		assert(minv <= res);
		assert(res <= maxv);
		return res;
	}

	long long readLong(long long minv, long long maxv) {
		assert(minv <= maxv);
		long long res = stoll(readOne());
		assert(minv <= res);
		assert(res <= maxv);
		return res;
	}

	void readSpace() {
		assert((int) buffer.size() > pos);
		assert(buffer[pos] == ' ');
		pos++;
	}

	void readEoln() {
		assert((int) buffer.size() > pos);
		assert(buffer[pos] == '\n');
		pos++;
	}

	void readEof() {
		assert((int) buffer.size() == pos);
	}
};

int main()
{
	ios::sync_with_stdio(false); cin.tie(0);

	input_checker inp;
	
	int t = inp.readInt(1, 10000); inp.readEoln();
	int sumN = 0;
	while (t--) {
		int n = inp.readInt(1, 100000); inp.readSpace();
		int x = inp.readInt(1, min(n, 26)); inp.readEoln();
		sumN += n;
		if (x > (n+1)/2) cout << -1 << '\n';
		else {
			for (int i = 0; i < x; ++i) cout << char('a'+i);
			for (int i = x; i < n - x; ++i) cout << 'a';
			for (int i = n-x+(2*x - 1 == n); i < n; ++i) cout << char('a'+n-i-1);
			cout << '\n';
		}
	}
	inp.readEof();
	assert(sumN <= 300000);
}