//Utkarsh.25dec #include <bits/stdc++.h> #define ll long long int #define pb push_back #define mp make_pair #define mod 1000000007 #define vl vector <ll> #define all(c) (c).begin(),(c).end() using namespace std; ll power(ll a,ll b) {ll res=1;a%=mod; assert(b>=0); for(;b;b>>=1){if(b&1)res=res*a%mod;a=a*a%mod;}return res;} ll modInverse(ll a){return power(a,mod-2);} const int N=500023; bool vis[N]; vector <int> adj[N]; long long readInt(long long l,long long r,char endd){ long long x=0; int cnt=0; int fi=-1; bool is_neg=false; while(true){ char g=getchar(); if(g=='-'){ assert(fi==-1); is_neg=true; continue; } if('0'<=g && g<='9'){ x*=10; x+=g-'0'; if(cnt==0){ fi=g-'0'; } cnt++; assert(fi!=0 || cnt==1); assert(fi!=0 || is_neg==false); assert(!(cnt>19 || ( cnt==19 && fi>1) )); } else if(g==endd){ if(is_neg){ x= -x; } if(!(l <= x && x <= r)) { cerr << l << ' ' << r << ' ' << x << '\n'; assert(1 == 0); } return x; } else { assert(false); } } } string readString(int l,int r,char endd){ string ret=""; int cnt=0; while(true){ char g=getchar(); assert(g!=-1); if(g==endd){ break; } cnt++; ret+=g; } assert(l<=cnt && cnt<=r); return ret; } long long readIntSp(long long l,long long r){ return readInt(l,r,' '); } long long readIntLn(long long l,long long r){ return readInt(l,r,'\n'); } string readStringLn(int l,int r){ return readString(l,r,'\n'); } string readStringSp(int l,int r){ return readString(l,r,' '); } void solve() { int n=readInt(1,100000,' '); int x=readInt(1,min(n,26),'\n'); if(n==1) { cout<<"a"<<'\n'; return; } if(x>((n+1)/2)) { cout<<-1<<'\n'; return; } string tmp=""; for(int i=0;i<x;i++) tmp+=('a'+i); while(tmp.length()<n/2) tmp+=('a'+x-1); string rev=""; for(int i=0;i<n/2;i++) rev+=tmp[i]; reverse(all(rev)); if(n%2==1 && tmp.length()<(n+1)/2) tmp+='a'; tmp+=rev; cout<<tmp<<'\n'; } int main() { #ifndef ONLINE_JUDGE freopen("input.txt", "r", stdin); freopen("output.txt", "w", stdout); #endif ios_base::sync_with_stdio(false); cin.tie(NULL),cout.tie(NULL); int T=readInt(1,10000,'\n'); while(T--) solve(); assert(getchar()==-1); cerr << "Time : " << 1000 * ((double)clock()) / (double)CLOCKS_PER_SEC << "ms\n"; }