#include <bits/stdc++.h>
#define N ('z'-'a')
using namespace std;
int n,ans;
string str[501];
long long mp[N+1];

void mk_mp(int pos,int i,int j){
  while(i<=j&&pos>=str[i].size())i++;
  if(i>=j)return;
  int ni=-1,ch=str[i][pos]-'a',c=1;

  while(i<j){
    int a=str[++i][pos]-'a',f=(ch==a);
    if(f&&ni==-1)ni=i-1;
    if(!f&&ni>=0)mk_mp(pos+1,ni,i-1),ni=-2;
    if(!f&&c)mk_mp(pos,i,j),mp[ch]|=((1LL<<a)|mp[a]),c=0;
    ans*=!(f&&!c)&!(f&&str[i-1].size()>str[i].size()&&!str[i][pos+1]);
  }
  if(ni>=0) mk_mp(pos+1,ni,j);
}

int main(){
  while(cin>>n,n){
    for(int i=0;i<n;i++)cin>>str[i];
    ans=1;
    mk_mp(0,0,n-1);
    for(int i=0;i<=N;mp[i]=0,i++) ans*=!(mp[i]&(1<<i));
    cout <<(ans? "yes":"no")<<endl;
  }
  return 0;
}