#include <iostream>
#include <sstream>
#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <cmath>
#include <ctime>
#include <vector>
#include <string>
#include <map>
#include <set>
#include <deque>
#include <queue>
#include <stack>
#include <functional>
#include <algorithm>

using namespace std;

#define rep(i,j) REP((i), 0, (j))
#define REP(i,j,k) for(int i=(j);(i)<(k);++i)
#define BW(a,x,b) ((a)<=(x)&&(x)<=(b))
#define ALL(v) (v).begin(), (v).end()
#define LENGTHOF(x) (sizeof(x) / sizeof(*(x)))
#define AFILL(a, b) fill((int*)a, (int*)(a + LENGTHOF(a)), b)
#define SQ(x) ((x)*(x))
#define Mod(x, mod) (((x)+(mod)%(mod))
#define MP make_pair
#define PB push_back
#define Fi first
#define Se second
#define INF (1<<29)
#define EPS 1e-10
#define MOD 1000000007

typedef pair<int, int> pi;
typedef pair<int, pi> pii;
typedef vector<int> vi;
typedef queue<int> qi;
typedef long long ll;

int n;
string S[512];
vector<int>G[32];

int dfs(int p, int d){
  if(d>26) return 0;
  for(auto g:G[p]){
    if(!dfs(g, d+1)) return 0;
  }
  return 1;
}

int solve()
{
  rep(i, 26) G[31].push_back(i);
  rep(i, n-1){
    //    cout << i << endl;
    int p = 0;
    string s=S[i], t = S[i+1];
    while(1){
      if(s[p] == '\0') break;
      if(t[p] == '\0') return 0;
      if(s[p] != t[p]){
	if(find(begin(G[s[p]-'a']), end(G[s[p]-'a']), t[p]-'a')
	   == end(G[s[p]-'a']))
	   G[s[p]-'a'].push_back(t[p]-'a');
	break;
      }
      p++;
    }
  }
  return dfs(31, 0);
}

int main()
{
  while(1){
    cin >> n;
    if(n == 0) return 0;
    rep(i, 32) G[i].clear();

    rep(i, n) cin >> S[i];

    cout << (solve()?"yes\n":"no\n");
  }
  return 0;
}