use std::collections::HashSet;

fn main() {
    match solve() {
        Some(ans) => println!("{}", ans),
        None => println!("-1"),
    }
}

fn solve() -> Option<String> {
    let n = {
        let mut line = String::new();
        std::io::stdin().read_line(&mut line).unwrap();
        line.trim().parse::<usize>().unwrap()
    };
    let mut ss: Vec<Vec<char>> = (0..n)
        .map(|_| {
            let mut line = String::new();
            std::io::stdin().read_line(&mut line).unwrap();
            line.trim().chars().collect()
        })
        .collect();

    ss.iter_mut().for_each(|s| s.sort_unstable());

    let mut used = HashSet::new();

    for s in &ss {
        for c in 'a'..='z' {
            let mut t = s.clone();
            t.push(c);
            t.sort_unstable();

            if used.contains(&t) {
                continue;
            }

            if ss.iter().filter(|s| is_match(&s, &t)).count() == 1 {
                return Some(t.iter().collect());
            }

            used.insert(t);
        }
    }

    None
}

fn is_match(s: &[char], t: &[char]) -> bool {
    if s.len() + 1 != t.len() {
        return false;
    }

    let mut counts1 = [0_u32; 26];
    for &c1 in s {
        counts1[char_to_number(c1)] += 1;
    }

    let mut counts2 = [0; 26];
    for &c2 in t {
        counts2[char_to_number(c2)] += 1;
    }

    counts1
        .iter()
        .zip(&counts2)
        .all(|(cnt1, cnt2)| cnt1 <= cnt2)
}

/// Converts the character `c` to the corresponding numeric value.
pub fn char_to_number(c: char) -> usize {
    (c as u8 - b'a') as usize
}