// implement 15min
#include <stdio.h>
#include <string.h>
#include <algorithm>
#include <iostream>
#include <math.h>
#include <assert.h>
#include <vector>
#include <queue>
#include <string>
#include <map>
#include <set>

using namespace std;
typedef long long ll;
typedef unsigned int uint;
typedef unsigned long long ull;
static const double EPS = 1e-9;
static const double PI = acos(-1.0);

#define REP(i, n) for (int i = 0; i < (int)(n); i++)
#define FOR(i, s, n) for (int i = (s); i < (int)(n); i++)
#define FOREQ(i, s, n) for (int i = (s); i <= (int)(n); i++)
#define FORIT(it, c) for (__typeof((c).begin())it = (c).begin(); it != (c).end(); it++)
#define MEMSET(v, h) memset((v), h, sizeof(v))

typedef int Weight;
struct Edge {
  int src;
  int dest;
  Weight weight;
  Edge(int src, int dest, Weight weight) : src(src), dest(dest), weight(weight) {;}
  bool operator<(const Edge &rhs) const {
    if (weight != rhs.weight) { return weight > rhs.weight; }
    if (src != rhs.src) { return src < rhs.src; }
    return dest < rhs.dest;
  }
};
typedef vector<Edge> Edges;
typedef vector<Edges> Graph;
typedef vector<Weight> Array;
typedef vector<Array> Matrix;

void PrintMatrix(const Matrix &matrix) {
  for (int y = 0; y < (int)matrix.size(); y++) {
    for (int x = 0; x < (int)matrix[y].size(); x++) {
      printf("%d ", matrix[y][x]);
    }
    puts("");
  }
}

void SccDfs(const Graph &g, int from, vector<int> &visit, vector<int> &st) {
  visit[from] = 1;
  for (Edges::const_iterator it = g[from].begin(); it != g[from].end(); it++) {
    if (visit[it->dest]) { continue; }
    SccDfs(g, it->dest, visit, st);
  }
  st.push_back(from);
}

vector<vector<int> > Scc(const Graph &g) {
  const int n = g.size();
  vector<vector<int> > ret;
  Graph revg(n);
  for (int i = 0; i < n; i++) {
    for (Edges::const_iterator it = g[i].begin(); it != g[i].end(); it++) {
      revg[it->dest].push_back(Edge(it->dest, i, it->weight));
    }
  }
  vector<int> st;
  vector<int> visit(n, 0);
  for (int i = 0; i < n; i++) {
    if (visit[i]) { continue; }
    SccDfs(g, i, visit, st);
  }
  visit = vector<int>(n, 0);
  for (int i = n - 1; i >= 0; i--) {
    int index = st[i];
    if (visit[index]) { continue; }
    vector<int> nret;
    SccDfs(revg, index, visit, nret);
    ret.push_back(nret);
  }
  return ret;
}

int n;
char strs[10010][20];
Graph g;

void CreateGraph(int depth, int start, int end) {
  if (end - start <= 1) { return; }
  int pstart = start;
  int prev = strs[start][depth];
  FOR(i, start, end) {
    int c = strs[i][depth];
    if (prev != c) {
      g[prev].push_back(Edge(prev, c, 0));
      CreateGraph(depth + 1, pstart, i);
      pstart = i;
      prev = c;
    }
  }
  if (prev != '\0') {
    CreateGraph(depth + 1, pstart, end);
  }
}

int main() {
  while (scanf("%d", &n) > 0 && n) {
    g = Graph(300);
    FOR(i, 1, 300) {
      g[0].push_back(Edge(0, i, 0));
    }
    REP(i, n) {
      scanf("%s", strs[i]);
    }
    CreateGraph(0, 0, n);
    vector<vector<int> > group = Scc(g);
    if ((int)group.size() == (int)g.size()) {
      puts("yes");
    } else {
      puts("no");
    }
  }
}