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

#define rep(i, a, b) for (int i = a; i < b; i++)
using ll = long long;

int main(){
	char s, e;
	int n;
	cin >> s >> e >> n;
	if(n == 1) {
		if(s == e) cout << s << endl;
		else cout << -1 << endl;
	}
	else {
		string t;
		t += s;
		int j = 0;
		rep(i, 0, n - 2) {
			char c = char(j + 'a');
			if(c == s || c == e) {
				j++;
				c = char(j + 'a');
			}
			if(c == s || c == e) {
				j++;
				c = char(j + 'a');
			}
			t += c;
			j++;
		}
		t += e;
		cout << t << endl;
	}
}