#include <bits/stdc++.h>
using namespace std;
bool les[26][26];
char s[105][15];
void analyse(char* x, char* y) {
  int len = min(strlen(x), strlen(y));
  for (int i = 0; i < len; ++i) {
    if (x[i] != y[i]) {
      les[x[i] - 'a'][y[i] - 'a'] = true;
      return;
    }
  }
}
bool illegal(char* x, char* y) {
  int l1 = strlen(x);
  int l2 = strlen(y);
  if (l1 <= l2) return false;
  for (int i = 0; i < l2; ++i) {
    if (x[i] != y[i]) {
      return false;
    }
  }
  return true;
}
int main() {
  int n;
  while (scanf("%d", &n) && n) {
    getchar();
    memset(les, 0, sizeof(les));
    for (int i = 0; i < n; ++i) {
      gets(s[i]);
    }
    for (int i = 0; i < n - 1; ++i) {
      for (int j = i + 1; j < n; ++j) {
        analyse(s[i], s[j]);
      }
    }
    for (int k = 0; k < 26; ++k) {
      for (int i = 0; i < 26; ++i) {
        for (int j = 0; j < 26; ++j) {
          if (les[i][k] && les[k][j]) {
            les[i][j] = true;
          }
        }
      }
    }
    bool flag = true;
    for (int i = 0; i < 26; ++i) {
      for (int j = i + 1; j < 26; ++j) {
        if (les[i][j] && les[j][i]) {
          flag = false;
        }
      }
    }
    for (int i = 0; i < n; ++i) {
      for (int j = i + 1; j < n; ++j) {
        if (illegal(s[i], s[j])) {
          flag = false;
        }
      }
    }
    if (flag)
      puts("yes");
    else
      puts("no");
  }
  return 0;
}
