use proconio::input;

fn main() {
    input! {
        s: char,
        e: char,
        n: usize,
    }

    if n == 1 {
        println!("-1");
    } else {
        let mid = std::iter::repeat_n('a', n - 2).collect::<String>();
        println!("{s}{mid}{e}");
    }
}