use proconio::input;
fn main() {
    input! {
        s: char,
        e: char,
        n: usize
    }
    if n == 1 && s != e {
        println!("-1");
    } else if n == 1 && s == e {
        println!("{}", s);
    } else {
        let x = "a".repeat(n - 2);
        println!("{}{}{}", s, x, e);
    }
}