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.*;
import java.util.*;
import java.lang.*;

public class Main {
    public static PrintWriter out = new PrintWriter(new BufferedOutputStream(System.out));
    static long MOD = (long) (1e9 + 7);
    //    static int MOD = 998244353;
    static long MOD2 = MOD * MOD;
    static FastReader sc = new FastReader();
    static int pInf = Integer.MAX_VALUE;
    static int nInf = Integer.MIN_VALUE;
    static long ded = (long)(1e17)+9;
    public static void main(String[] args) throws Exception {
        int test = 1;
        test = sc.nextInt();
        for (int i = 1; i <= test; i++){
//            out.print("Case #"+i+": ");
            solve();
        }
        out.flush();
        out.close();
    }
    static char[] c;
    static int n,k;
    static void solve()throws Exception{
        n = sc.nextInt();
        k = sc.nextInt();
        c = sc.next().toCharArray();
        if(n%k!=0){
            out.println(-1);
            return;
        }
        int l = -1;
        int r = n;
        char[] ans = new char[n];
        while(r-l>1){
            int m = l+(r-l)/2;
            temp = new char[n];
            if(good(m)){
                l = m;
                ans = temp;
            }else{
                r = m;
            }
        }
        out.println(ans);
    }
    static char[] temp;
    static boolean good(int idx){
        int[] f = new int[26];
        temp = new char[n];
        Arrays.fill(temp,'0');
        int r = n-1;
        for(int i = 0; i <= idx; i++){
            temp[i] = c[i];
            f[c[i]-'a']++;
        }
        if(idx+1<n){
            if(c[idx+1]=='z'){
                temp[idx+1] = c[idx+1];
                f[c[idx+1]-'a']++;
            }else{
                char g = c[idx+1];
                g++;
                temp[idx+1] = g;
                f[temp[idx+1]-'a']++;
            }
        }
        for(int i= 25; i >=0; i--){
            if(f[i]%k!=0){
                int res = k-(f[i]%k);
                while(res-->0&&r>=0){
                    if(temp[r]!='0'){
                        return false;
                    }
                    temp[r] = (char) ('a'+i);
                    r--;
                    f[i]++;
                }
            }
        }
        for(int i= 0; i < n; i++){
            if(temp[i]=='0'){
                temp[i] = 'a';
                f[0]++;
            }
        }
        for(int i =0; i < 26; i++){
            if(f[i]%k!=0){
                return false;
            }
        }
        return true;
    }
    static class Pair implements Comparable<Pair> {
        int x;
        int y;
        int idx;
        public Pair(int x, int y) {
            this.x = x;
            this.y = y;
        }
        @Override
        public int compareTo(Pair o) {
            return -1*(this.x-o.x);
        }
    }
    public static long mul(long a, long b) {
        return ((a % MOD) * (b % MOD)) % MOD;
    }
    public static long add(long a, long b) {
        return ((a % MOD) + (b % MOD)) % MOD;
    }
    public static long c2(long n) {
        if ((n & 1) == 0) {
            return mul(n / 2, n - 1);
        } else {
            return mul(n, (n - 1) / 2);
        }
    }
    //Shuffle Sort
    static final Random random = new Random();
    static void ruffleSort(int[] a) {
        int n = a.length;//shuffle, then sort
        for (int i = 0; i < n; i++) {
            int oi = random.nextInt(n); int temp= a[oi];
            a[oi] = a[i];
            a[i] = temp;
        }
        Arrays.sort(a);
    }
    //Brian Kernighans Algorithm
    static long countSetBits(long n) {
        if (n == 0) return 0;
        return 1 + countSetBits(n & (n - 1));
    }
    //Euclidean Algorithm
    static long gcd(long A, long B) {
        if (B == 0) return A;
        return gcd(B, A % B);
    }
    //Modular Exponentiation
    static long fastExpo(long x, long n) {
        if (n == 0) return 1;
        if ((n & 1) == 0) return fastExpo((x * x) % MOD, n / 2) % MOD;
        return ((x % MOD) * fastExpo((x * x) % MOD, (n - 1) / 2)) % MOD;
    }
    //AKS Algorithm
    static boolean isPrime(long n) {
        if (n <= 1) return false;
        if (n <= 3) return true;
        if (n % 2 == 0 || n % 3 == 0) return false;
        for (int i = 5; i <= Math.sqrt(n); i += 6)
            if (n % i == 0 || n % (i + 2) == 0) return false;
        return true;
    }
    public static long modinv(long x) {
        return modpow(x, MOD - 2);
    }
    public static long modpow(long a, long b) {
        if (b == 0) {
            return 1;
        }
        long x = modpow(a, b / 2);
        x = (x * x) % MOD;
        if (b % 2 == 1) {
            return (x * a) % MOD;
        }
        return x;
    }

    static class FastReader {
        BufferedReader br;
        StringTokenizer st;
        public FastReader() {
            br = new BufferedReader(new InputStreamReader(System.in));
        }

        String next() {
            while (st == null || !st.hasMoreElements()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        }

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

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

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

        String nextLine() {
            String str = "";
            try {
                str = br.readLine();
            } catch (IOException e) {
                e.printStackTrace();
            }
            return str;
        }
    }
}