// >>> TEMPLATES
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using i32 = int32_t;
using i64 = int64_t;
using u32 = uint32_t;
using u64 = uint64_t;
#define int ll
#define double ld
#define rep(i,n) for (int i = 0; i < (int)(n); i++)
#define rep1(i,n) for (int i = 1; i <= (int)(n); i++)
#define repR(i,n) for (int i = (int)(n)-1; i >= 0; i--)
#define rep1R(i,n) for (int i = (int)(n); i >= 1; i--)
#define loop(i,a,B) for (int i = a; i B; i++)
#define loopR(i,a,B) for (int i = a; i B; i--)
#define all(x) (x).begin(), (x).end()
#define allR(x) (x).rbegin(), (x).rend()
#define pb push_back
#define eb emplace_back
#define mp make_pair
#define fst first
#define snd second
template <class Int> auto constexpr inf = numeric_limits<Int>::max()/2-1;
auto constexpr INF32 = inf<int32_t>;
auto constexpr INF64 = inf<int64_t>;
auto constexpr INF   = inf<int>;
#ifdef LOCAL
#include "debug.hpp"
#else
#define dump(...) (void)(0)
#define say(x) (void)(0)
#define debug if (0)
#endif
template <class T> using pque_max = priority_queue<T>;
template <class T> using pque_min = priority_queue<T, vector<T>, greater<T> >;
template <class T, class = typename T::iterator, class = typename enable_if<!is_same<T, string>::value>::type>
ostream& operator<<(ostream& os, T const& v) { bool f = true; for (auto const& x : v) os << (f ? "" : " ") << x, f = false; return os; }
template <class T, class = typename T::iterator, class = typename enable_if<!is_same<T, string>::value>::type>
istream& operator>>(istream& is, T &v) { for (auto& x : v) is >> x; return is; }
template <class T, class S> ostream& operator<<(ostream& os, pair<T,S> const& p) { return os << "(" << p.first << ", " << p.second << ")"; }
template <class T, class S> istream& operator>>(istream& is, pair<T,S>& p) { return is >> p.first >> p.second; }
struct IOSetup { IOSetup() { cin.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(15); } } iosetup;
template <class F> struct FixPoint : private F {
    constexpr FixPoint(F&& f) : F(forward<F>(f)) {}
    template <class... T> constexpr auto operator()(T&&... x) const { return F::operator()(*this, forward<T>(x)...); }
};
struct MakeFixPoint {
    template <class F> constexpr auto operator|(F&& f) const { return FixPoint<F>(forward<F>(f)); }
};
#define MFP MakeFixPoint()|
#define def(name, ...) auto name = MFP [&](auto &&name, __VA_ARGS__)
template <class T, size_t d> struct vec_impl {
    using type = vector<typename vec_impl<T,d-1>::type>;
    template <class... U> static type make_v(size_t n, U&&... x) { return type(n, vec_impl<T,d-1>::make_v(forward<U>(x)...)); }
};
template <class T> struct vec_impl<T,0> { using type = T; static type make_v(T const& x = {}) { return x; } };
template <class T, size_t d = 1> using vec = typename vec_impl<T,d>::type;
template <class T, size_t d = 1, class... Args> auto make_v(Args&&... args) { return vec_impl<T,d>::make_v(forward<Args>(args)...); }
template <class T> void quit(T const& x) { cout << x << endl; exit(0); }
template <class T, class U> constexpr bool chmin(T& x, U const& y) { if (x > y) { x = y; return true; } return false; }
template <class T, class U> constexpr bool chmax(T& x, U const& y) { if (x < y) { x = y; return true; } return false; }
template <class It> constexpr auto sumof(It b, It e) { return accumulate(b,e,typename iterator_traits<It>::value_type{}); }
template <class T> int sz(T const& x) { return x.size(); }
template <class C, class T> int lbd(C const& v, T const& x) {
    return lower_bound(v.begin(), v.end(), x)-v.begin();
}
template <class C, class T> int ubd(C const& v, T const& x) {
    return upper_bound(v.begin(), v.end(), x)-v.begin();
}
const int dx[] = { 1,0,-1,0 };
const int dy[] = { 0,1,0,-1 };
constexpr int popcnt(ll x) { return __builtin_popcountll(x); }
template <class Int> struct Random {
    mt19937_64 mt{random_device{}()};
    //mt19937_64 mt{(unsigned)time(0)};
    Int a,b; // [a,b]
    Random(Int a, Int b) : a(a), b(b) {}
    Int operator()() { return uniform_int_distribution<Int>(a,b)(mt); }
};
template <class Int> Int rand(Int a, Int b) { // [a,b]
    static mt19937_64 mt{random_device{}()};
    return uniform_int_distribution<Int>(a,b)(mt);
}
// <<<
// >>> scc

constexpr int dst(int v) { return v; }
template <class E, class = decltype(E().to)>
constexpr int dst(E const& e) { return e.to; }

template <class E> struct SCC {
    vector<vector<E> > const& g;
    vector<int32_t> ord, low, st;
    int32_t time = 0, pos = 0, sz = 0;
    SCC(vector<vector<E> > const& g) : g(g), ord(g.size()), low(g.size()), st(g.size()) {
        rep (v,g.size()) if (ord[v] == 0) dfs(v);
        for (auto &x : ord) x += sz;
    }
    void dfs(int v) {
        low[v] = ord[v] = ++time; st[pos++] = v;
        for (auto const& e : g[v]) {
            if (ord[dst(e)] == 0) {
                dfs(dst(e));
                chmin(low[v], low[dst(e)]);
            } else if (ord[dst(e)] > 0) {
                chmin(low[v], ord[dst(e)]);
            }
        }
        if (low[v] == ord[v]) {
            sz++; do ord[st[--pos]] = -sz; while (st[pos] != v);
        }
    }

    // t-sorted id
    int id(int v) const { return ord[v]; }
    int size() const { return sz; }
    vector<vector<int>> groups() const {
        vector<vector<int> > grp(sz);
        rep (v,g.size()) grp[ord[v]].push_back(v);
        return grp;
    }

    // no multiple edges, no self-loops
    // no edge information
    vector<vector<int>> graph() const {
        vector<vector<int>> ret(size());
        rep (v,g.size()) {
            for (auto const& e : g[v]) {
                int x = id(v), y = id(dst(e));
                if (x != y) ret[x].push_back(y);
            }
        }
        for (auto &ls : ret) {
            sort(ls.begin(), ls.end());
            ls.erase(unique(ls.begin(), ls.end()), ls.end());
        }
        return ret;
    }
};
template <class E> auto get_scc(vector<vector<E> > const& g) { return SCC<E>(g); }

// <<<

void solve(int n) {
    vector<string> ss(n); cin >> ss;

    vector<vector<int>> g(26);
    rep (i,n-1) {
        auto s = ss[i], t = ss[i+1];
        int j = 0;
        for ( ; j < min(sz(s),sz(t)); ++j) {
            if (s[j] != t[j]) {
                g[s[j]-'a'].eb(t[j]-'a');
                break;
            }
        }
        if (j == min(sz(s),sz(t))) {
            if (sz(s) == sz(t)) continue;
            if (sz(s) > sz(t)) {
                cout << "no\n";
                return;
            }
        }
    }

    auto scc = get_scc(g);
    cout << (scc.size() == 26 ? "yes" : "no") << "\n";

}

int32_t main() {
    while (true) {
        int n; cin >> n;
        if (n == 0) break;
        solve(n);
    }
}



Measured memory_bytes: 3144