#include<bits/stdc++.h> using namespace std; int pvs[100005]; int nxs[100005]; int nxt[100005]; int fs[130]; int ls[130]; int cnt[130]; int bs[100005]; int bsl[100005]; int k,n,t; string s; int main(){ //freopen("subseq.in","r",stdin); //freopen("subseq.out","r",stdin); cin>>t; while(t--){ cin>>k; cin>>s; n=s.size(); s=" "+s; int cn=0; char minn=128; for(int i = 1;i<=n;i++){ pvs[s[i]]=ls[s[i]]; nxt[pvs[s[i]]]=i; ls[s[i]]=i; if(!fs[s[i]]) fs[s[i]]=i,cn++; minn=min(minn,s[i]); } if(k>cn){ cout<<-1<<endl; } for(int i = 1;i<=n;i++){ if(cnt[s[i]]>1&&s[i]>=s[i+1])bs[i]=1,cnt[s[i]]--; } for(int i = 1;i<=k;i++){ for(int j = 128;j>=1;j--){ if(cnt[j]>0){ cnt[j]=0; bsl[j]=1; break; } } } for(int i = 1;i<=n;i++){ if(bs[i])continue; if(bsl[s[i]])continue; printf("%c",s[i]); } printf("\n"); } }