#include<bits/stdc++.h>
using namespace std;
template<typename AA>inline void CMax(AA &u,AA v){if(v>u)u=v;}
template<typename AA>inline void CMin(AA &u,AA v){if(v<u)u=v;}
#define Put(a) cout<<(a)<<endl
#define For(a,b,c) for(int a=(b);a<=(c);++a)
#define Fov(a,b,c) for(int a=(b);a>=(c);--a)
#define Foe(a,b,c) for(int a=(b);a<(c);++a)
#define Clear(a,b) memset(a,b,sizeof(a))
const double eps=1E-8;const int Inf=0x3f3f3f3f,oo=0x7fffffff,Mod=998244353;
typedef long long lon;const lon Lnf=0x3f3f3f3f3f3f3f3f;
template<typename AA>void read(AA &x)
{int s=0;char c;while(c=getchar(),(c<'0'||c>'9')&&c!='-');c=='-'?(s=1,x=0):(x=c-'0');
while(c=getchar(),c>='0'&&c<='9') x=x*10+c-'0';if(s) x=-x;}
template<typename AA, typename...Args>void read(AA &t,Args &...args)
{read(t);read(args...);}template<typename AA>void print(AA k){int num=0;
static char stk[23];if(k<0){putchar('-');k=-k;}while(k>0)stk[++num]=k%10+48,k/=10;
if(!num)putchar('0');else while(num)putchar(stk[num--]);putchar('\n');}
inline int Log2(int x) {if(!x)return 0;else return 31-__builtin_clz(x);}
char getc(){char ch;do{ch=getchar();}while(ch==' '||ch=='\n');return ch;}
lon Pow(lon d,lon m){lon f=1;while(m){if(m&1)f=f*d%Mod;d=d*d%Mod;m>>=1;}return f;}
//#include<ext/rope> using namespace __gnu_cxx;
//#include<bits/extc++.h>#include<ext/pb_ds/assoc_container.hpp>
//using namespace __gnu_pbds;
//#pragma GCC optimize(3)
int N,M,S,Q,K,T=1;

int u,v;
char s[300030];

int n;
int nd[300030];
int ansp;char ansc;

void init() {
	int cnt[26]={};
	For(a,1,n) {
		if(cnt[s[a]-'a']==0) nd[a]=nd[a-1]+K-1;
		else nd[a]=nd[a-1]-1;

		cnt[s[a]-'a']=(cnt[s[a]-'a']+1)%K;

		if(a<n&&s[a+1]<'z') {
			Fov(b,25,0) {
				if('a'+b==s[a+1]) break;

				if(cnt[b]==0) {
					nd[a+1]=nd[a]+K-1;
				}
				else {
					nd[a+1]=nd[a]-1;
				}

				if(n-a-1>=nd[a+1]&&(n-a-1-nd[a+1])%K==0) ansp=a+1,ansc='a'+b;

			}
		}

	}
}


signed main()
{
	//freopen("1.in","r",stdin);

	read(T);
	while(T--) {
		read(N,K);
		scanf("%s",s+1); n=strlen(s+1);
		ansp=1;ansc=s[1]+1;
		if(N%K) {puts("-1");continue;}

		init();
		
		//printf("%d %c\n",ansp,ansc);

		if(nd[n]==0) {printf("%s\n",s+1);continue;}
		if(ansp==1&&ansc>'z') {puts("-1");continue;}

		

		int cnt[26]={};
		s[ansp]=ansc;
		For(a,1,ansp) {
			cnt[s[a]-'a']=(cnt[s[a]-'a']+1)%K;
		}
		For(a,0,25) if(cnt[a]!=0) cnt[a]=K-cnt[a];

		int now=n;
		Fov(a,25,0) {
			while(cnt[a]--) s[now--]='a'+a;
		}
		while(now>ansp) s[now--]='a';

		printf("%s\n",s+1);
	}


	return 0;
}