#include <bits/stdc++.h>
#define mp make_pair
#define fi first
#define se second
#define pb push_back
#define eb emplace_back
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define forn(i, n) for (int i = 0; i < (int)(n); ++i)
#define for1(i, n) for (int i = 1; i <= (int)(n); ++i)
#define ms(x, y) memset(x, y, sizeof(x))
#define SZ(x) int(x.size())
#define fk cout<<"fk"<<endl
#define db(x) cout<<(#x)<<'='<<(x)<<endl
#define db2(x,y) cout<<(#x)<<'='<<(x)<<' '<<(#y)<<'='<<(y)<<endl
using namespace std;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef long long i64;
template<class T> bool uin(T &a, T b) { return a > b ? (a = b, true) : false; }
template<class T> bool uax(T &a, T b) { return a < b ? (a = b, true) : false; }
void fastio(){ios::sync_with_stdio(false); cin.tie(0); cout.precision(10); cout<<fixed;}
//1.integer overflow (1e5 * 1e5) (2e9 + 2e9)
//2.runtime error
//3.boundary condition
int tc,n,k,cnt[30],need[30];
string s;
int main() {
    fastio();

    cin>>tc;
    while(tc--){
        cin>>n>>k;
        cin>>s;
        ms(cnt,0);
        for(int i=0; i<n; ++i){
            int num=int(s[i])-int('a');
            ++cnt[num];
        }
        bool ok=true;
        for(int i=0; i<26; ++i){
            if(cnt[i]%k!=0) ok=false;
        }
        if(ok){
            cout<<s<<'\n';
            continue;
        }
        bool have=false;
        string ans="";
        for(int i=n-1; i>=0; --i){
            string res="";
            int cur=int(s[i])-int('a');
            --cnt[cur];
            if(cur==25) continue;
            int all_need=0;
            ms(need,0);
            for(int j=0; j<26; ++j){
                int goal=(cnt[j]+k-1)/k*k;
                need[j]=goal-cnt[j];
                all_need+=goal-cnt[j];
            }
            if(all_need<=n-i){
                if(all_need==n-i){
                    int find=-1;
                    for(int j=cur+1; j<26; ++j){
                        if(need[j]>0){
                            find=j;
                            break;
                        }
                    }
                    if(find!=-1){
                        have=true;
                        for(int j=0; j<i; ++j) res+=s[j];
                        char ch=char(int('a')+find);
                        res+=ch;
                        --need[find];
                        for(int j=0; j<26; ++j){
                            while(need[j]>0){
                                char ch_temp=char(int('a')+j);
                                res+=ch_temp;
                                need[j]--;
                            }
                        }
                        ans=res;
                        break;
                    }
                } else{
                    if(((n-i)-all_need)%k==0){
                        int find=-1;
                        for(int j=cur+1; j<26; ++j){
                            if(need[j]>0){
                                find=j;
                                break;
                            }
                        }
                        if(find==cur+1){
                            need[0]+=(n-i)-all_need;
                            have=true;
                            for(int j=0; j<i; ++j) res+=s[j];
                            res+=char(int('a')+cur+1);
                            --need[cur+1];
                            for(int j=0; j<26; ++j){
                                while(need[j]>0){
                                    res+=char(int('a')+j);
                                    --need[j];
                                }
                            }
                            ans=res;
                            break;
                        } else{
                            need[cur+1]+=k;
                            need[0]+=(n-i)-all_need-k;
                            have=true;
                            for(int j=0; j<i; ++j) res+=s[j];
                            res+=char(int('a')+cur+1);
                            --need[cur+1];
                            for(int j=0; j<26; ++j){
                                while(need[j]>0){
                                    res+=char(int('a')+j);
                                    --need[j];
                                }
                            }
                            ans=res;
                            break;
                        }
                    }
                }
            }
        }
     //   if(have) assert(SZ(ans)==n);
        if(have) cout<<ans<<'\n';
        else cout<<-1<<'\n';
    }
    return 0;
}
