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.util.*;
import java.io.*;
import java.math.*;
import java.text.*;

public class Main {
    static PrintWriter out;
    static Reader in;
    public static void main(String[] args) throws IOException {
        input_output();
        Main solver = new Main();
        solver.solve();
        out.close(); 
        out.flush(); 
    }

    static long INF = (long)1e18;
    static int MAXN = (int)1e7+5;
    static int mod = 998_244_353;
    static int n, m, q, t;
    static double pi = Math.PI;

    void solve() throws IOException{
    	t = in.nextInt();

    	loop:while (t --> 0) {
    		n = in.nextInt();
    		int k = in.nextInt();
    		String s = in.next();

    		int[] cnt = new int[26];
    		for (int i = 0; i < n ; i++) cnt[s.charAt(i)-'a']++;

    		boolean good = true;
    		for (int i = 0; i < 26; i++) {
    			if (cnt[i]%k != 0) good = false;
    		}

    		if (good) {
    			out.println(s);
    			continue;
    		}
    		
    		int[] put = new int[26];
    		for (int i = n-1; i >= 0; i--) {
    			cnt[s.charAt(i)-'a']--;
    			int need = 0;
    			for (int j = s.charAt(i)-'a'+1; j < 26; j++) {
    				cnt[j]++;
    				good = true;
    				need = 0;
    				for (int ii = 0; ii < 26; ii++) {
    					need += (k-(cnt[ii]%k))%k;
    					put[ii] = (k-(cnt[ii]%k))%k;
    				}

    				if (need > n-i || (n-(i+1)-need)%k != 0) good = false;
    				
    				if (good) {
    					put[0] += (n-(i+1)-need);
    					printans(s, i, put, j);
    					continue loop;
    				} else {
    					cnt[j]--;
    				}
    			}
    		}

    		out.println(-1);
    	}
   	}

    //<>

   	static void printans(String s, int ii, int[] put, int c) {
   		StringBuilder ans = new StringBuilder("");
   		for (int i = 0; i < ii; i++) ans.append(s.charAt(i));
   		ans.append((char)(c+'a'));

   		for (int i = 0; i < 26; i++) {
   			while (put[i] != 0) {
   				put[i]--;
   				ans.append((char)(i+'a'));
   			}
   		}
   		out.println(ans);
   	}

    static class Reader {
 
        private InputStream mIs;
        private byte[] buf = new byte[1024];
        private int curChar;
        private int numChars;
    
        public Reader() {
            this(System.in);
        }
    
        public Reader(InputStream is) {
            mIs = is;
        }
    
        public int read() {
            if (numChars == -1) {
                throw new InputMismatchException();
    
        }
            if (curChar >= numChars) {
                curChar = 0;
                try {
                    numChars = mIs.read(buf);
                } catch (IOException e) {
                    throw new InputMismatchException();
                }
                if (numChars <= 0) {
                    return -1;
                }
            }
            return buf[curChar++];
        }
    
        public String nextLine() {
            int c = read();
            while (isSpaceChar(c)) {
                c = read();
            }
            StringBuilder res = new StringBuilder();
            do {
                res.appendCodePoint(c);
                c = read();
            } while (!isEndOfLine(c));
            return res.toString();
        }
    
        public String next() {
            int c = read();
            while (isSpaceChar(c)) {
                c = read();
            }
            StringBuilder res = new StringBuilder();
            do {
                res.appendCodePoint(c);
                c = read();
            } while (!isSpaceChar(c));
            return res.toString();
        }
    
        double nextDouble()
        {
            return Double.parseDouble(next());
        }
    
        public long nextLong() {
            int c = read();
            while (isSpaceChar(c)) {
                c = read();
            }
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = read();
            }
            long res = 0;
            do {
                if (c < '0' || c > '9') {
                    throw new InputMismatchException();
                }
                res *= 10;
                res += c - '0';
                c = read();
            } while (!isSpaceChar(c));
            return res * sgn;
        }
    
        public int nextInt() {
            int c = read();
            while (isSpaceChar(c)) {
                c = read();
            }
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = read();
            }
            int res = 0;
            do {
                if (c < '0' || c > '9') {
                    throw new InputMismatchException();
                }
                res *= 10;
                res += c - '0';
                c = read();
            } while (!isSpaceChar(c));
            return res * sgn;
        }
    
        public boolean isSpaceChar(int c) {
            return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }
    
        public boolean isEndOfLine(int c) {
            return c == '\n' || c == '\r' || c == -1;
        }
 
    }
    static void input_output() throws IOException {
        File f = new File("in.txt");
        if(f.exists() && !f.isDirectory()) { 
            in = new Reader(new FileInputStream("in.txt"));
        } else in = new Reader();
        f = new File("out.txt");
        if(f.exists() && !f.isDirectory()) {
            out = new PrintWriter(new File("out.txt"));
        } else out = new PrintWriter(System.out);
    }
}