#include <bits/stdc++.h>   using namespace std;   #define me(a, b) memset(a, b, sizeof(a))   #define IOS() ios::sync_with_stdio(false), cin.tie(0)   #define endl   n       typedef long long ll;   typedef pair<ll, ll> pll;   typedef pair<int, int> pii;   const int INF = 0x3f3f3f3f;   const int maxn = 2e5 + 5;   const ll mod = 1e9 + 7;      int cnt[26];   int n, k;   vector<pii> ans;   int check()   {    ans.clear();    int sum = 0;    for(int i = 0; i < 26; ++i) {     int r = cnt[i] % k;     if(r){      ans.push_back(pii(i, k - r));      sum += k - r;     }    }    return sum;   }      void solve()   {    me(cnt, 0);    string s;    cin >> n >> k >> s;    for(char &c : s)     ++cnt[c -  a ];    if(check() == 0) {     cout << s << endl;     return ;    }    for(int i = s.size()-1; i >= 0; --i) {     char &c = s[i];     --cnt[c -  a ];     for(int j = s[i] -  a  + 1; j < 26; ++j) {      c = j +  a ;      ++cnt[j];      int sum = s.size()-i-1 - check();      --cnt[j];      if(sum >= 0 && sum % k == 0) {       string t;       for(int p = 0; p <= i; ++p)        t.push_back(s[p]);       t.append(sum,  a );       for(pii p : ans) {        t.append(p.second, p.first+ a );       }       cout << t << endl;       return ;      }        }    }    cout <<  -1 n ;   }         int main()   {    IOS();    int T;    cin >> T;    while(T--)     solve();    return 0;   }