#![allow(non_snake_case, unused_imports)]

use proconio::{input, marker::Usize1, marker::Chars};
use itertools::Itertools;

#[allow(unused_macros)]
macro_rules! d {
    ( $( $x:expr ),* $(,)? ) => {
        eprintln!(
            concat!( $( stringify!($x), "={:?} " ),* ),
            $( $x ),*
        );
    };
}

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

    if N < 2 {
        if s == e {
            println!("{}", s);
        } else {
            println!("-1");
        }
    } else {
        let mut ans = String::new();
        ans.push(s);
        ans.push_str(&"a".repeat(N-2));
        ans.push(e);

        println!("{}", ans);
    }
}