#ifdef NACHIA
#define _GLIBCXX_DEBUG
#else
// disable assert
#define NDEBUG
#endif
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
using namespace std;
using ll = long long;
const ll INF = 1ll << 60;
#define REP(i,n) for(ll i=0; i<ll(n); i++)
template <class T> using V = vector<T>;
template <class A, class B> void chmax(A& l, const B& r){ if(l < r) l = r; }
template <class A, class B> void chmin(A& l, const B& r){ if(r < l) l = r; }

void testcase(){
    char s, e; cin >> s >> e;
    ll N; cin >> N;
    if(N == 1){
        if(s == e){
            cout << s << "\n";
        } else {
            cout << "-1\n";
        }
    } else {
        cout << s << string(N-2, 'a') << e << "\n";
    }
}

int main(){
  cin.tie(0)->sync_with_stdio(0);
  testcase();
  return 0;
}