12
12 3
zazasqzqsasq
1 1
z
1 1
a
15 4
abccbaabccbazaa
16 8
cadjfdsljfdkljds
6 2
baazaa
6 3
bosszb
6 1
qwerty
7 7
daaaaaa
14 2
acabcbdeffewzd
18 6
zzzzzzzzzzzzyyyyyz
9 4
aabaabbbc

zazasqzqsasq
z
a
-1
cbbbbbbbbccccccc
baazbz
bpbbpp
qwerty
ddddddd
acabcbdeffexdx
zzzzzzzzzzzzzzzzzz
-1

import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.math.BigInteger;
import java.util.*;

/**
 * #
 *
 * @author pttrung
 */
public class C_Round_705_Div2 {
    public static long MOD = 998244353;
    static int last;


    public static void main(String[] args) throws FileNotFoundException {
        // PrintWriter out = new PrintWriter(new FileOutputStream(new File(
        // "output.txt")));
        PrintWriter out = new PrintWriter(System.out);
        Scanner in = new Scanner();
        int T = in.nextInt();
        for (int z = 1; z <= T; z++) {
            int n = in.nextInt();
            int k = in.nextInt();
            char[] data = in.next().toCharArray();
            int[] count = new int[26];
            for (int i = 0; i < n; i++) {
                int index = data[i] - 'a';
                count[index]++;
            }
            boolean ok = true;

            for (int i : count) {
                if (i % k != 0) {
                    ok = false;
                    break;
                }
            }
            if (ok) {
                out.println(data);
                continue;
            }
            for (int i = n - 1; i >= 0; i--) {
                int cur = data[i] - 'a';
                count[cur]--;
                if (data[i] == 'z') {
                    continue;
                }
                int left = n - i;
                int need = 0;
                for (int j : count) {
                    need += (k - (j % k)) % k;
                }

                ok = true;
                if (need <= left && (left - need) % k == 0) {
                    int index = -1;
                    int nxt = data[i] - 'a' + 1;
                    if (left > need || (count[nxt] % k) != 0) {
                        index = nxt;
                    } else {
                        for (int j = nxt; j < count.length; j++) {
                            if ((count[j] % k) != 0) {
                                index = j;
                                break;
                            }
                        }
                    }
                    if (index == -1) {
                        ok = false;
                        continue;
                    }
                    count[index]++;
                    data[i] = (char) (index + 'a');
                    need = 0;
                    for (int j : count) {
                        need += (k - (j % k)) % k;
                    }
                    for (int j = i + 1; j < n - need; j++) {
                        data[j] = 'a';
                    }
                    for (int j = n - need; j < n; j++) {
                        for (int h = 0; h < count.length; h++) {
                            if (count[h] % k != 0) {
                                count[h]++;
                                data[j] = (char) (h + 'a');
                                break;
                            }
                        }
                    }
                    break;
                }
                ok = false;
            }
            if (!ok) {
                out.println(-1);
            } else {
                out.println(new String(data));
            }

        }
        out.close();
    }


    public static int[] KMP(String val) {
        int i = 0;
        int j = -1;
        int[] result = new int[val.length() + 1];
        result[0] = -1;
        while (i < val.length()) {
            while (j >= 0 && val.charAt(j) != val.charAt(i)) {
                j = result[j];
            }
            j++;
            i++;
            result[i] = j;
        }
        return result;

    }

    public static boolean nextPer(int[] data) {
        int i = data.length - 1;
        while (i > 0 && data[i] < data[i - 1]) {
            i--;
        }
        if (i == 0) {
            return false;
        }
        int j = data.length - 1;
        while (data[j] < data[i - 1]) {
            j--;
        }
        int temp = data[i - 1];
        data[i - 1] = data[j];
        data[j] = temp;
        Arrays.sort(data, i, data.length);
        return true;
    }

    public static int digit(long n) {
        int result = 0;
        while (n > 0) {
            n /= 10;
            result++;
        }
        return result;
    }

    public static double dist(long a, long b, long x, long y) {
        double val = (b - a) * (b - a) + (x - y) * (x - y);
        val = Math.sqrt(val);
        double other = x * x + a * a;
        other = Math.sqrt(other);
        return val + other;

    }

    public static class Point implements Comparable<Point> {

        long x;
        int y;

        public Point(long start, int end) {
            this.x = start;
            this.y = end;
        }

        @Override
        public boolean equals(Object o) {
            if (this == o) return true;
            if (o == null || getClass() != o.getClass()) return false;
            Point point = (Point) o;
            return x == point.x &&
                    y == point.y;
        }

        @Override
        public int hashCode() {
            return Objects.hash(x, y);
        }

        @Override
        public int compareTo(Point o) {
            return Long.compare(x, o.x);
        }
    }

    public static class FT {

        long[] data;

        FT(int n) {
            data = new long[n];
        }

        public void update(int index, long value) {
            while (index < data.length) {
                data[index] += value;
                index += (index & (-index));
            }
        }

        public long get(int index) {
            long result = 0;
            while (index > 0) {
                result += data[index];
                index -= (index & (-index));
            }
            return result;

        }
    }

    public static long gcd(long a, long b) {
        if (b == 0) {
            return a;
        }
        return gcd(b, a % b);
    }

    public static long pow(long a, long b) {
        if (b == 0) {
            return 1;
        }
        if (b == 1) {
            return a;
        }

        long val = pow(a, b / 2);
        if (b % 2 == 0) {
            return (val * val) % MOD;
        } else {
            return (val * ((val * a) % MOD)) % MOD;

        }
    }

    static class Scanner {

        BufferedReader br;
        StringTokenizer st;

        public Scanner() throws FileNotFoundException {
            // System.setOut(new PrintStream(new BufferedOutputStream(System.out), true));
            br = new BufferedReader(new InputStreamReader(System.in));
            //br = new BufferedReader(new InputStreamReader(new FileInputStream(new File("input.txt"))));
        }

        public String next() {

            while (st == null || !st.hasMoreTokens()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (Exception e) {
                    throw new RuntimeException();
                }
            }
            return st.nextToken();
        }

        public long nextLong() {
            return Long.parseLong(next());
        }

        public int nextInt() {
            return Integer.parseInt(next());
        }

        public double nextDouble() {
            return Double.parseDouble(next());
        }

        public String nextLine() {
            st = null;
            try {
                return br.readLine();
            } catch (Exception e) {
                throw new RuntimeException();
            }
        }

        public boolean endLine() {
            try {
                String next = br.readLine();
                while (next != null && next.trim().isEmpty()) {
                    next = br.readLine();
                }
                if (next == null) {
                    return true;
                }
                st = new StringTokenizer(next);
                return st.hasMoreTokens();
            } catch (Exception e) {
                throw new RuntimeException();
            }
        }
    }
}