#include<iostream>
using namespace std;
int main(){
    string s, e;
    int n;
    cin >> s >> e >> n;
    if(n == 1){
        if(s == e){
            cout << s << endl;
        }else{
            cout << -1 << endl;
        }
    }else{
        cout << s;
        for(int i=0; i<n-2; i++){ cout << "a"; }
        cout << e << endl;
    }
}