#include <bits/stdc++.h>
using namespace std;
#define rep(i,a,b) for(ll i=a;i<b;i++)
#define rrep(i,a,b) for(ll i=a-1;i>=b;i--)
#define ll long long
#define ull unsigned ll
#define ld long double
#define bl __int128_t
#define fi first
#define se second
#define vel vector<ll>
#define vvel vector<vel>
#define pll pair<ll,ll>
#define vepll vector<pll>
#define vvepll vector<vepll>
#define ves vector<string>
#define vem vector<mint>
#define vvem vector<vem>
#define pmm pair<mint,mint>
#define cleout(i) cout<<fixed<<setprecision(i)
template<class T>using PQ=priority_queue<T,vector<T>,greater<T>>;
//               上  右 下 左
vector<int> di={-1, 0, 1, 0};
vector<int> dj={ 0, 1, 0,-1};

vector<int> dx={ 0, 1, 0,-1};
vector<int> dy={ 1, 0,-1, 0};


vector<int> ddx={ 1, 1, 1, 0, -1, -1, -1, 0 };
vector<int> ddy={ 1, 0, -1, -1, -1, 0, 1, 1 };

ll inf=1000000000000000000;//1e18
// LLONG_MAX

mt19937_64 rng((ull)chrono::steady_clock::now().time_since_epoch().count());

//[x^M]1/(1-x)^N=comb(N-1+M,M)

struct hoge{
    vel next;//aho_corasickの遷移先
    ll failure;//遷移先がない時にどこまで戻るか(trie木に含まれる文字列=suffixとなる最長の場所のindex)
    ll output;//自分未満で初めてヒットするやつ
    ll deep;//この頂点の深さ
    ll P;//親のindex
    vel ids;
    ll cnt;
    hoge(ll dp,ll var){
        next.assign(var,-1);
        deep=dp;
        P=0;
        failure=0;
        output=0;
        cnt=0;
    }
};


struct trie{
    vector<hoge> node;
    char base='a';
    ll var;
    vel id;
    trie(ll v=26):var(v){//この頂点の子供の数
        hoge x(0,var);
        node.push_back(x);
    }
    ll insert(string &S,ll num){
        ll now=0;
        rep(i,0,S.size()){
            node[now].cnt++;
            if(node[now].next[S[i]-base]==-1){
                hoge x(i+1,var);
                node[now].next[S[i]-base]=node.size();
                node.push_back(x);
            }
            ll next=node[now].next[S[i]-base];
            node[next].P=now;
            now=node[now].next[S[i]-base];
            if(i+1==S.size()){
                node[now].ids.push_back(num);
                id.push_back(now);
                //node[now].ids|=(1ll<<num);//bit用
            }
        }
        node[now].cnt++;
        return now;
    }

    void solve(string &ans,ll now,ll K,bool &ok){
        if(ok)return ;
        if(node[now].cnt<=K){
            cout<<"Yes\n";
            cout<<ans<<"\n";
            ok=1;
            return ;
        }
        if(node[now].ids.size()>K)return ;
        rep(i,0,26){
            ans+=(char)('a'+i);
            if(node[now].next[i]==-1){
                cout<<"Yes\n";
                cout<<ans<<"\n";
                ok=1;
                return ;
            }else{
                solve(ans,node[now].next[i],K-node[now].ids.size(),ok);
                if(ok)return ;
            }
            ans.pop_back();
        }
    }
};

void _solve(){
    ll N,K;
    cin>>N>>K;
    trie tree;
    rep(i,0,N){
        string S;
        cin>>S;
        tree.insert(S,i);
    }
    string ans="";
    bool ok=0;
    tree.solve(ans,0,N-K,ok);
    if(!ok){
        cout<<"No\n";
    }
}


int main(){
    cin.tie(nullptr);
 	ios_base::sync_with_stdio(false);

    
    ll _;
    bool multitest=0;
    if(multitest)cin>>_;
    else _=1;
    rep(__,0,_){
        _solve();
    }
}