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

typedef unsigned int uint;
typedef long long int ll;
typedef unsigned long long ull;
typedef long double ld;

#define loop while(1)
#define rep(i,n) for(ll i=0;(i)<(n);(i)++)
#define REP(i,a,n) for(ll i=(a);(i)<(n);(i)++)
#define modi(a,b) (a%b+b)%b
#define all(x) x.begin(), x.end()
#define debug(x) cerr << #x << " = " << x << endl
#define dout()  cout << fixed << setprecision(20)

int main(){
	string s, e;
	ll N;
    cin >> s >> e >> N;
    if(N == 1){
    	if (s == e) cout << s << endl;
    	else cout << -1 << endl;
    	exit(0);
    }
    cout << s;
    N--;
    N--;
    while(N--) cout << "a";
    cout << e << endl;
}