#include <bits/stdc++.h>
using namespace std;

int main() {
    char s, e;
    int N;
    cin >> s >> e >> N;

    if (N == 1) {
        if (s == e) cout << s << endl;
        else cout << -1 << endl;
        return 0;
    }

    string ans = "";
    ans += s;

    for (int i = 0; i < N - 2; i++) {
        ans += 'a';
    }

    ans += e;

    cout << ans << endl;

    return 0;
}