#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int inf = 1 << 30;
const ll INF = 1LL << 60;
const int dx[] = {1, 0, -1, 0}, dy[] = {0, 1, 0, -1};
#define rep(i, s, t) for (ll i = (ll)s; i < (ll)(t); i++)
#define rrep(i, s, t) for (ll i = (ll)(t) - 1; i >= (ll)(s); i--)
#define all(x) begin(x), end(x)
#define rall(x) rbegin(x), rend(x)

template <class T1, class T2>
bool chmin(T1& x, T2 y) { return x > y ? (x = y, true) : false; }
template <class T1, class T2>
bool chmax(T1& x, T2 y) { return x < y ? (x = y, true) : false; }

template <class T>
using min_heap = priority_queue<T, vector<T>, greater<T>>;
template <class T>
using max_heap = priority_queue<T>;

void solve() {
    char c, s;
    int N;
    cin >> c >> s >> N;
    if (N == 1) {
        if (c == s) cout << c << endl;
        else cout << -1 << endl;
    } else {
        cout << c;
        rep(i, 0, N - 2) cout << 'a';
        cout << s << endl;
    }
}

int main() {
    cin.tie(0)->sync_with_stdio(0);
    cout << fixed << setprecision(15);
    srand(time(NULL));

    int T;
    // cin >> T;
    T = 1;
    while (T--) solve();
}