#pragma GCC optimize("Ofast,unroll-loops,no-stack-protector")
#pragma GCC target("avx2,bmi2,lzcnt,popcnt")
#include <bits/stdtr1c++.h>
#include <bits/stdc++.h>
#include <bits/extc++.h>
#include <atcoder/all>
using namespace std;
using namespace __gnu_pbds;
using namespace atcoder;
using ll=long long;
using ld=long double;
constexpr ll inf=LLONG_MAX/4;
constexpr ll mod=998244353;
constexpr ld eps=1e-9;
constexpr ld pi=acosl(-1);
static mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
#define GET_MACRO(_1,_2,_3,NAME,...) NAME
#define rep2(i,n) for(ll i=0,_end_##i=(n);i<_end_##i;i++)
#define rep3(i,a,b) for(ll i=(a),_end_##i=(b);i<_end_##i;i++)
#define rep(...) GET_MACRO(__VA_ARGS__,rep3,rep2)(__VA_ARGS__)
#define rrep2(i,n) for(ll _beg_##i=0,i=(n);i-->_beg_##i;)
#define rrep3(i,a,b) for(ll _beg_##i=(a),_end_##i=(b),i=_end_##i;i-->_beg_##i;)
#define rrep(...) GET_MACRO(__VA_ARGS__,rrep3,rrep2)(__VA_ARGS__)
#define pb push_back
#define mp make_pair
#define all(x) (x).begin(),(x).end()
#define rall(x) (x).rbegin(),(x).rend()
#define name(x,...) x(#x,##__VA_ARGS__)
template<std::size_t N,class T>struct vec_pow{using type=vector<typename vec_pow<N-1,T>::type>;};
template<class T>struct vec_pow<0,T>{using type=T;};
template<size_t N,class T>using vN=typename vec_pow<N,T>::type;
template<class T>using v=vN<1,T>;
template<class T>using vv=vN<2,T>;
template<class T>using vvv=vN<3,T>;
template<class T,class U>ostream&operator<<(ostream&os,const pair<T,U>&p){return os<<"("<<p.first<<","<<p.second<<")";}
template<class T,class U>istream&operator>>(istream&is,pair<T,U>&p){return is>>p.first>>p.second;}
template<class T>ostream&operator<<(ostream&os,const vector<T>&v){rep(i,v.size()){if(i)os<<" ";os<<v[i];}return os;}
template<class T>istream&operator>>(istream&is,vector<T>&v){rep(i,v.size()){is>>v[i];}return is;}
template<class T>ostream&operator<<(ostream&os,const set<T>&s){auto it=s.begin();while(it!=s.end()){if(it!=s.begin())os<<" ";os<<*it;it++;}return os;}
template<class T>ostream&operator<<(ostream&os,const multiset<T>&s){auto it=s.begin();while(it!=s.end()){if(it!=s.begin())os<<" ";os<<*it;it++;}return os;}
template<class T,class U>ostream&operator<<(ostream&os,const map<T,U>&m){os<<"{";ll f=1;auto it=m.begin();while(it!=m.end()){if(!f){os<<",";}os<<it->first<<":"<<it->second;it++;f=0;}return os<<"}";}
template<typename... T>istream&operator>>(istream&is,tuple<T...>&t){apply([&is](auto&&... args){((is>>args),...);},t);return is;}
void print(){cout<<'\n';}
template<class T,class... U>void print(const T&t,const U&...u){cout<<t;if(sizeof...(u)){cout<<' ';}print(u...);}
void dbg_out(){cerr<<endl;}
template<typename H,typename... T>void dbg_out(H h,T... t){cerr<<" "<<h;dbg_out(t...);}
#define dbg(...) cerr<<"["<<#__VA_ARGS__<<"]:",dbg_out(__VA_ARGS__)
template<typename T>struct dbga{string n;T v;dbga(string n,T v=T()):n(n),v(v){}dbga&operator=(const T&nv){cerr<<"[upd] "<<n<<":"<<v<<"->"<<nv<<endl;v=nv;return*this;}dbga&operator+=(const T&x){T o=v;v+=x;cerr<<"[add] "<<n<<":"<<o<<"->"<<v<<endl;return*this;}dbga&operator-=(const T&x){T o=v;v-=x;cerr<<"[sub] "<<n<<":"<<o<<"->"<<v<<endl;return*this;}operator T()const{return v;}};
template<typename T>ostream&operator<<(ostream&os,const dbga<T>&d){return os<<d.v;}
template<typename T>istream&operator>>(istream&is,dbga<T>&d){T t;is>>t;d=t;return is;}
template<typename U>struct dbga<vector<U>>{string n;vector<U>v;dbga(string n,int s,U d=U()):n(n),v(s,d){}dbga(string n,vector<U>i):n(n),v(i){}dbga&operator=(const vector<U>&nv){cerr<<"[updall] "<<n<<" sz:"<<v.size()<<"->"<<nv.size()<<endl;v=nv;return*this;}struct pxy{dbga*p;ll i;pxy&operator=(const U&x){cerr<<"[upd] "<<p->n<<"["<<i<<"]:"<<p->v[i]<<"->"<<x<<endl;p->v[i]=x;return*this;}pxy&operator+=(const U&x){U o=p->v[i];p->v[i]+=x;cerr<<"[add] "<<p->n<<"["<<i<<"]:"<<o<<"->"<<p->v[i]<<endl;return*this;}operator U()const{return p->v[i];}friend istream&operator>>(istream&is,pxy p){U x;is>>x;p=x;return is;}};pxy operator[](ll i){return pxy{this,i};}const U&operator[](ll i)const{return v[i];}size_t size()const{return v.size();}void push_back(U x){v.pb(x);cerr<<"[push] "<<n<<" val:"<<x<<" sz->"<<v.size()<<endl;}auto begin(){return v.begin();}auto end(){return v.end();}operator vector<U>()const{return v;}};
template<typename U>ostream&operator<<(ostream&os,const dbga<vector<U>>&d){return os<<d.v;}
template<typename U>istream&operator>>(istream&is,dbga<vector<U>>&d){vector<U>t=d.v;is>>t;d=t;return is;}
struct grid{ll h,w;const ll dy[4]={0,0,1,-1};const ll dx[4]={1,-1,0,0};grid(ll h,ll w):h(h),w(w){}ll ok(ll r,ll c)const{return 0<=r&&r<h&&0<=c&&c<w;}ll id(ll r,ll c)const{return r*w+c;}pair<ll,ll>crd(ll i)const{return{i/w,i%w};}vector<pair<ll,ll>>adj(ll r,ll c)const{vector<pair<ll,ll>>v;rep(i,4){ll nr=r+dy[i],nc=c+dx[i];if(ok(nr,nc)){v.pb({nr,nc});}}return v;}vector<v<ll>>to_g(const vector<string>&s,char wall='#')const{vector<v<ll>>g(h*w);rep(r,h){rep(c,w){if(s[r][c]==wall){continue;}ll u=id(r,c);rep(i,4){ll nr=r+dy[i],nc=c+dx[i];if(ok(nr,nc)&&s[nr][nc]!=wall){g[u].pb(id(nr,nc));}}}}return g;}};
// ----- End of Template -----
int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.setf(ios::fixed);
    char s,k;
    ll n;
    cin>>s>>k>>n;
    if(n>=2){
    	print(s+string(n-2,'a')+k);
    }
    else{
    	if(s==k){
    		print(s);
    	}
    	print(-1);
    }
    return 0;
}