#include <iostream>

#include <cstring>

#include <cstdio>

#define L 1111111

#define LL long long

using namespace std;

const int Mod=1e9+9,Upbit=4373;//1111111111111111111

int tim,len;

char st[L];

LL hsp[L],hsp2[L];

LL pubh,sths;

LL lva,rva;

LL ans=0;

LL ppow(LL a,LL b){

	LL res=1;

	a%=Mod;

	b%=Mod-1;

	while(b){

		if(b&1)res=res*a%Mod;

		a=a*a%Mod;

		b>>=1;

	}

	return res;

}

inline LL tur(const char ch){

	return ch-'A'+1;

}

int main(){

//	freopen("ccx.in" ,"r",stdin);\

//	freopen("ccx.out","w",stdout);

	hsp[0]=1;

	for(int i=1;i<=1000000;i++)

		hsp[i]=hsp[i-1]*Upbit%Mod;

	int T;

	cin>>T;

	while(T--){

		memset(hsp2,0,sizeof hsp2);

		ans=sths=pubh=0;

		scanf("%d%d%s",&tim,&len,st+1);

//		puts("Inputend");

		for(int i=1;i<=len;i++)

			sths=(sths*Upbit%Mod+tur(st[i]))%Mod;

//		puts("String hash end");

		hsp2[0]=ppow(Upbit,1ll*len*(tim-1));

		for(int i=1;i<=len;i++)

			hsp2[i]=hsp2[i-1]*Upbit%Mod;

//		puts("Hash tim*len end");

		for(int i=1;i<tim;i++)

			pubh=(pubh*hsp[len]%Mod+sths)%Mod;

//		puts("One Hash end");

		rva=lva=pubh;

		ans=1ll*len*(tim-1);

//		puts("Start Che!");

		for(int r=1,l=len;r<len;r++,l--){

			rva=(rva*Upbit%Mod+tur(st[r]))%Mod;//Add

			lva=(lva+hsp2[r-1]*tur(st[l])%Mod)%Mod;

//			cout<<r<<" "<<l<<" "<<rva<<" "<<lva<<endl;

			if(rva==lva)

				ans=max(ans,1ll*r+1ll*len*(tim-1));

//			cout<<ans<<endl;

		}

//		puts("end Che!");

		if(ans==0)ans=-1;

		printf("%lld\n",ans);

	}

}