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

int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);

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