#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  string a[505];
  while (scanf("%d", &n) != EOF && n) {
    int ans = 0;
    for (int i = 0; i < n; i++) {
      cin >> a[i];
      if (i > 0) {
        for (int j = 0; j < i; j++) {
          int flag = 0;
          int k = 0;
          while (a[i][k]) {
            if (a[i][k] != a[j][k]) {
              flag = 1;
              break;
            }
            k++;
          }
          if (flag == 0) {
            ans = 0;
            break;
          } else
            ans = 1;
        }
      }
    }
    if (ans == 0)
      cout << "no" << endl;
    else
      cout << "yes" << endl;
  }
  return 0;
}
