#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
using mint = modint998244353;
//using mint = modint1000000007;
typedef long long ll;
typedef pair<ll, ll> P;
typedef tuple<ll, ll, ll> T;
template<typename T>bool chmax(T& a, const T& b) { if (a < b) { a = b;return true; } else { return false; } }
template<typename T>bool chmin(T& a, const T& b) { if (a > b) { a = b;return true; } else { return false; } }

#ifdef LOCAL
template<class T, class U>ostream& operator<<(ostream& o, const pair<T, U>& p) { return o << "(" << p.first << ", " << p.second << ")"; }
template<class T>auto operator<<(ostream& o, const T& v)->decltype(v.begin(), o) { o << "{";int c = 0;for (auto& x : v)o << (c++ ? ", " : "") << x;return o << "}"; }
#define dbg(...) cerr<<"["<<#__VA_ARGS__<<"]: ",([](auto&&... a){((cerr<<a<<' '),...);})(__VA_ARGS__),cerr<<'\n'
#else
#define dbg(...) void(0)
#endif

const int di[] = { -1,0,1,0 };
const int dj[] = { 0,-1,0,1 };
const int inf = 0x3f3f3f3f;
const long long INF = 0x3f3f3f3f3f3f3f3fLL;


int main(void) {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    char s, e;cin >> s >> e;
    int n; cin >> n;
    if (n == 1) {
        cout << -1 << "\n";
        return 0;
    }
    cout << s;
    for (int i = 0; i < n - 2; i++) {
        cout << 'a';
    }
    cout << e;
    cout << "\n";
}