import java.util.*; 
import java.io.*;
import java.math.*;
import java.util.stream.*;
import java.util.function.*;

class Main {

    public final PrintWriter out = new PrintWriter(System.out);
    public final FastScanner in = new FastScanner();
    public static final int inf = (1 << 30) ;
    public static final long lnf = (1L << 30);
    public boolean dbg = false ;
    
    public void solve() {
        int n = in.nextInt();
        Map<String,Integer> memo = new HashMap<>();
        String [] str = new String[n];
        List<String> us = new ArrayList<>();
        Map<String,Integer> cnt = new HashMap<>();
        for(int i = 0 ; i < n ; i ++) {
            str[i] = in.next();
            cnt.put(str[i] , cnt.getOrDefault(str[i] , 0) + 1);
        }
        for(String sss : cnt.keySet()) {
            if(cnt.get(sss) >= 2) continue;
            List<Character> ss = new ArrayList<>();
            for(int j = 0 ; j < sss.length() ; j ++) ss.add(sss.charAt(j));
            Collections.sort(ss);
            StringBuilder sb = new StringBuilder();
            for(char c : ss) sb.append(c);
            String res = sb.toString();
            memo.put(res , memo.getOrDefault(res , 0) + 1);
        }
        String ans = null ;
        for(String qq : memo.keySet()) if(memo.get(qq) < 2) ans = qq ;
        out.println(ans == null ? -1 : ans + 'a');

        out.flush();
    }  

    public static void main(String [] args) { 
        new Main().solve();
    }



}

final class Utils {

    public static final int [] dy4 = {-1, 0, 1, 0} , dx4 = {0, 1, 0, -1} ;
    public static final int [] dy8 = {-1, -1, -1, 0, 1, 1, 1, 0} , dx8 = {-1, 0, 1, 1, 1, 0, -1, -1} ;

    public boolean isNumber(char c) {
        return '0' <= c && c <= '9';
    }

    public boolean isNumber(String s) {
        for(int i = 0 ; i < s.length() ; i ++) if(!isNumber(s.charAt(i))) return false;
        return true ;
    }

    public boolean isInclude(int l , int r , int L , int R) {
        return (L <= l && r <= R) ;
    }

    public boolean isInclude_2D(int y , int x , int h , int w) {
        return (0 <= y && y < h) && (0 <= x && x < w);
    }

    public char Character_shift(char c , int s) {
        if('a' <= c && c <= 'z') return (char) (((((c - 'a') + s) % 26) + 26) % 26 + 'a');
        if('A' <= c && c <= 'Z') return (char) (((((c - 'A') + s) % 26) + 26) % 26 + 'A');
        if('0' <= c && c <= '9') return (char) (((((c - '0') + s) % 10) + 10) % 10 + '0');
        return 'c';
    }

    // ボクシング用

    public Integer [] toInteger(int [] a) {
        return Arrays.stream(a).boxed().toArray(Integer[]::new);
    }

    public Long [] toLong(long [] a) {
        return Arrays.stream(a).boxed().toArray(Long[]::new);
    }

    public Double [] toDouble(double [] a) {
        return Arrays.stream(a).boxed().toArray(Double[]::new);
    }

    public <T> List<T> ArrayList(T [] a) { 
        return Arrays.stream(a).collect(Collectors.toCollection(ArrayList::new));
    }

    public <T> HashSet<T> HashSet(T [] a) { 
        return Arrays.stream(a).collect(Collectors.toCollection(HashSet::new));
    }

    public <T> TreeSet<T> TreeSet(T [] a) { 
        return Arrays.stream(a).collect(Collectors.toCollection(TreeSet::new));
    }

    public <T> Deque<T> Deque(T [] a) { 
        return Arrays.stream(a).collect(Collectors.toCollection(ArrayDeque::new));
    }

    public <T> PriorityQueue<T> PriorityQueue(T [] a) { 
        return Arrays.stream(a).collect(Collectors.toCollection(PriorityQueue::new));
    }

    // コレクション配列

    public <T> List <T> [] ArrayList(int n) { 
        @SuppressWarnings("unchecked")
        List<T> [] G = new ArrayList[n]; 
        for(int i = 0 ; i < n ; i ++ ) G[i] = new ArrayList<>();
        return G ; 
    }

    public <T> HashSet <T> [] HashSet(int n) { 
        @SuppressWarnings("unchecked")
        HashSet<T> [] G = new HashSet[n]; 
        for(int i = 0 ; i < n ; i ++ ) G[i] = new HashSet<>()  ;
        return G ; 
    }

    public <T> TreeSet <T> [] TreeSet(int n) { 
        @SuppressWarnings("unchecked")
        TreeSet<T> [] G = new TreeSet[n]; 
        for(int i = 0 ; i < n ; i ++ ) G[i] = new TreeSet<>()  ; 
        return G ; 
    }

    public <T> TreeSet <T> [] TreeSet(int n , Comparator<? super T> comparator) { 
        @SuppressWarnings("unchecked")
        TreeSet<T> [] G = new TreeSet[n]; 
        for(int i = 0 ; i < n ; i ++ ) G[i] = new TreeSet<>(comparator)  ;
        return G ; 
    }

    public <T> Deque <T> [] Deque(int n) { 
        @SuppressWarnings("unchecked")
        Deque<T> [] G = new ArrayDeque[n]; 
        for(int i = 0 ; i < n ; i ++ ) G[i] = new ArrayDeque<>(); 
        return G ; 
    }

    public <T> PriorityQueue <T> [] PriorityQueue(int n) { 
        @SuppressWarnings("unchecked")
        PriorityQueue<T> [] G = new PriorityQueue[n]; 
        for(int i = 0 ; i < n ; i ++ ) G[i] = new PriorityQueue<>(); 
        return G ; 
    }

    public <T> PriorityQueue <T> [] PriorityQueue(int n , Comparator<? super T> comparator) { 
        @SuppressWarnings("unchecked")
        PriorityQueue<T> [] G = new PriorityQueue[n]; 
        for(int i = 0 ; i < n ; i ++ ) G[i] = new PriorityQueue<>(comparator); 
        return G ; 
    }

    public <K,V> HashMap <K,V> [] HashMap(int n) { 
        @SuppressWarnings("unchecked")
        HashMap<K,V> [] G = new HashMap[n];
        for(int i = 0 ; i < n ; i ++ ) G[i] = new HashMap<>()  ;
        return G ; 
    }

    public <K,V> TreeMap <K,V> [] TreeMap(int n) { 
        @SuppressWarnings("unchecked")
        TreeMap<K,V> [] G = new TreeMap[n];
        for(int i = 0 ; i < n ; i ++ ) G[i] = new TreeMap<>()  ;
        return G ;
    }

    public <K,V> TreeMap <K,V> [] TreeMap(int n , Comparator<? super K> comparator) { 
        @SuppressWarnings("unchecked")
        TreeMap<K,V> [] G = new TreeMap[n];
        for(int i = 0 ; i < n ; i ++ ) G[i] = new TreeMap<>(comparator)  ;
        return G ; 
    }

}

final class FastScanner {

    private final InputStream in = System.in;
    private final Scanner sc = new Scanner(System.in);
    private final byte[] buffer = new byte[1024];
    private int ptr = 0;
    private int buflen = 0;

    private boolean hasNextByte() {
        if (ptr < buflen) {
            return true;
        }else{
            ptr = 0;
            try {
                buflen = in.read(buffer);
            } catch (IOException e) {
                e.printStackTrace();
            }
            if (buflen <= 0) {
                return false;
            }
        }
        return true;
    }

    private int readByte() { 
        if (hasNextByte()) return buffer[ptr++]; else return -1;
    }

    private static boolean isPrintableChar(int c) { 
        return 33 <= c && c <= 126;
    }

    private boolean hasNext() { 
        while(hasNextByte() && !isPrintableChar(buffer[ptr])) {
            ptr++; 
        }
        return hasNextByte();
    }
    
    public String next() {
        if (!hasNext()) throw new NoSuchElementException();
        StringBuilder sb = new StringBuilder();
        int b = readByte();
        while(isPrintableChar(b)) {
            sb.appendCodePoint(b);
            b = readByte();
        }
        return sb.toString();
    }

    public long nextLong() {
        if (!hasNext()) throw new NoSuchElementException();
        long n = 0;
        boolean minus = false;
        int b = readByte();
        if (b == '-') {
            minus = true;
            b = readByte();
        }
        if (b < '0' || '9' < b) {
            throw new NumberFormatException();
        }
        while(true){
            if ('0' <= b && b <= '9') {
                n *= 10;
                n += b - '0';
            }else if(b == -1 || !isPrintableChar(b)){
                return minus ? -n : n;
            }else{
                throw new NumberFormatException();
            }
            b = readByte();
        }
    }

    public int nextInt() {
        long nl = nextLong();
        if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE) throw new NumberFormatException();
        return (int) nl;
    }

    public double nextDouble() { 
        return Double.parseDouble(next());
    }

    public char nextChar() {
        return next().charAt(0);
    }

    public BigInteger nextBigInteger() {
        return sc.nextBigInteger();
    }

    public int [] nextInt(int n) {
        int [] array = new int[n];
        for(int i = 0 ; i < n ; i ++) {
            array[i] = nextInt();
        }
        return array ;
    }

    public int [][] nextInt(int n , int m) {
        int [][] array = new int[n][m];
        for(int i = 0 ; i < n ; i ++) {
            array[i] = nextInt(m);
        }
        return array ;
    }

    public long [] nextLong(int n) {
        long [] array = new long[n];
        for(int i = 0 ; i < n ; i ++) {
            array[i] = nextLong();
        }
        return array ;
    }

    public long [][] nextLong(int n , int m) {
        long [][] array = new long[n][m];
        for(int i = 0 ; i < n ; i ++) {
            array[i] = nextLong(m);
        }
        return array ;
    }

    public double [] nextDouble(int n) {
        double [] array = new double[n];
        for(int i = 0 ; i < n ; i ++) {
            array[i] = nextDouble();
        }
        return array ;
    }
    
    public String [] next(int n) {
        String [] array = new String[n];
        for(int i = 0 ; i < n ; i ++) {
            array[i] = next();
        }
        return array ;
    }

    public String [][] next(int n , int m) {
        String [][] array = new String[n][m];
        for(int i = 0 ; i < n ; i ++) {
            array[i] = next(m);
        }
        return array ;
    }

    public char [] nextChar(int n) {
        return next().toCharArray();
    }

    public char [][] nextChar(int n , int m) {
        char [][] array = new char[n][m];
        for(int i = 0 ; i < n ; i ++) {
            array[i] = nextChar(m);
        }
        return array ;
    }

}