import java.io.*;
import java.util.*;

public class Main {

    static int N;
    static String[] S;

    public static void main(String[] args) {
        FastScanner sc = new FastScanner(System.in);

        while(true) {
            N = sc.nextInt();
            if( N == 0 ) break;

            S = new String[N];
            for (int i = 0; i < N; i++) {
                S[i] = sc.next();
            }

            System.out.println(solve() ? "yes" : "no");
        }
    }

    static boolean solve() {
        // a < b -> dic[a][b] = -1
        int[][] dic = new int[26][26];
        lo:
        for (int i = 0; i < N-1; i++) {
            String s = S[i];
            String t = S[i+1];

            int n = Math.min(s.length(), t.length());
            for (int j = 0; j < n; j++) {
                int a = s.charAt(j) - 'a';
                int b = t.charAt(j) - 'a';

                if( a == b ) continue;

                if( dic[a][b] == 0 ) {
                    dic[a][b] = -1;
                    dic[b][a] = 1;
                    continue lo;

                } else if( dic[a][b] == -1 ) {
                    continue lo;

                } else {
                    return false;
                }
            }

            // 最後まで同じだったので長さ勝負
            if( s.length() > t.length() ) return false;
        }

        // 循環チェック
        List<Edge> E = new ArrayList<>();
        for (int i = 0; i < 26; i++) {
            for (int j = 0; j < 26; j++) {
                if( dic[i][j] == -1 ) {
                    E.add( new Edge(i, j) );
                }
            }
        }

        int[][] G = adjD(26, E);
        return khan(26, G) != null;
    }

    static int[] khan(int V, int[][] G) {
        int[] deg = new int[V];
        for (int[] tos : G) {
            for (int to : tos) {
                deg[to]++;
            }
        }

        int[] q = new int[V];
        int a = 0, b = 0;
        for (int v = 0; v < V; v++) {
            if( deg[v] == 0 ) q[b++] = v;
        }

        int[] ret = new int[V];
        int idx = 0;
        while( a != b ) {
            int v = q[a++];
            ret[idx++] = v;
            for (int to : G[v]) {
                deg[to]--;
                if( deg[to] == 0 ) {
                    q[b++] = to;
                }
            }
        }

        for (int v = 0; v < V; v++) {
            if( deg[v] != 0 ) return null;
        }
        return ret;
    }

    static class Edge {
        int a, b;

        public Edge(int a, int b) {
            this.a = a;
            this.b = b;
        }
    }

    static int[][] adjD(int n, List<Edge> es) {
        int[][] adj = new int[n][];
        int[] cnt = new int[n];

        for (Edge e : es) {
            cnt[e.a]++;
        }
        for (int i = 0; i < n; i++) {
            adj[i] = new int[cnt[i]];
        }
        for (Edge e : es) {
            adj[e.a][--cnt[e.a]] = e.b;
        }
        return adj;
    }

    @SuppressWarnings("unused")
    static class FastScanner {
        private BufferedReader reader;
        private StringTokenizer tokenizer;

        FastScanner(InputStream in) {
            reader = new BufferedReader(new InputStreamReader(in));
            tokenizer = null;
        }

        String next() {
            if (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

        String nextLine() {
            if (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    return reader.readLine();
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken("\n");
        }

        long nextLong() {
            return Long.parseLong(next());
        }

        int nextInt() {
            return Integer.parseInt(next());
        }

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

        int[] nextIntArray(int n, int delta) {
            int[] a = new int[n];
            for (int i = 0; i < n; i++) a[i] = nextInt() + delta;
            return a;
        }

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

    static void writeLines(int[] as) {
        PrintWriter pw = new PrintWriter(System.out);
        for (int a : as) pw.println(a);
        pw.flush();
    }

    static void writeLines(long[] as) {
        PrintWriter pw = new PrintWriter(System.out);
        for (long a : as) pw.println(a);
        pw.flush();
    }

    static void writeSingleLine(int[] as) {
        PrintWriter pw = new PrintWriter(System.out);
        for (int i = 0; i < as.length; i++) {
            if (i != 0) pw.print(" ");
            pw.print(as[i]);
        }
        pw.println();
        pw.flush();
    }

    static int max(int... as) {
        int max = Integer.MIN_VALUE;
        for (int a : as) max = Math.max(a, max);
        return max;
    }

    static int min(int... as) {
        int min = Integer.MAX_VALUE;
        for (int a : as) min = Math.min(a, min);
        return min;
    }

    static void debug(Object... args) {
        StringJoiner j = new StringJoiner(" ");
        for (Object arg : args) {
            if (arg == null) j.add("null");
            else if (arg instanceof int[]) j.add(Arrays.toString((int[]) arg));
            else if (arg instanceof long[]) j.add(Arrays.toString((long[]) arg));
            else if (arg instanceof double[]) j.add(Arrays.toString((double[]) arg));
            else if (arg instanceof Object[]) j.add(Arrays.toString((Object[]) arg));
            else j.add(arg.toString());
        }
        System.err.println(j.toString());
    }

    static void printSingleLine(int[] array) {
        PrintWriter pw = new PrintWriter(System.out);
        for (int i = 0; i < array.length; i++) {
            if (i != 0) pw.print(" ");
            pw.print(array[i]);
        }
        pw.println();
        pw.flush();
    }

    static int lowerBound(int[] array, int value) {
        int lo = 0, hi = array.length, mid;
        while (lo < hi) {
            mid = (hi + lo) / 2;
            if (array[mid] < value) lo = mid + 1;
            else hi = mid;
        }
        return lo;
    }

    static int upperBound(int[] array, int value) {
        int lo = 0, hi = array.length, mid;
        while (lo < hi) {
            mid = (hi + lo) / 2;
            if (array[mid] <= value) lo = mid + 1;
            else hi = mid;
        }
        return lo;
    }
}



Measured memory_bytes: 30684