#include <bits/stdc++.h>
#define REP(i,n) for(int i=0; i<(int)(n); ++i)

using namespace std;

int main(){
    int N;
    while(cin >> N && N){
        bool ok = true;
        vector<string> words(N);
        REP(i, N) cin >> words[i];
        bool g[30][30] = {};
        for(int i = 0; i < N; i++){
            for(int j = i + 1; j < N; j++){
                for(int k = 0; k < words[i].size(); k++){
                    if(k == words[j].size()){
                        ok = false;
                        break;
                    }
                    if(words[i][k] != words[j][k]){
                        g[ words[i][k] - 'a' ][ words[j][k] - 'a' ] = true;
                        break;
                    }
                }
            }
        }
        REP(k, 26) REP(i, 26) REP(j, 26) g[i][j] |= g[i][k] & g[k][j];
        REP(i, 26) if(g[i][i]) ok = false;
        cout << (ok ? "yes" : "no") << endl;
    }
    return 0;
}