#include <bits/stdc++.h>
using namespace std;

struct Trie{
    struct Node {
        std::array<int,26> child;
        int sz, hik;
        Node(void) :sz(0), hik(0) { child.fill(-1); }
        const int& operator[](int k) const { return child[k]; }
        int& operator[](int k) { return child[k]; }
    };
    int num;
    vector<Node> tree;
    Node& operator[](int k) { return tree[k]; }
    Trie() : num(0) {
        tree.push_back(Node());
    }
    void insert(const string &s){
        int cur = 0;
        for(auto c: s){
            tree[cur].sz++;
            if(tree[cur][c - 'a'] == -1){
                tree[cur][c - 'a'] = ++num;
                tree.push_back(Node());
            }
            cur = tree[cur][c - 'a'];
        }
        tree[cur].sz++;
        tree[cur].hik++;
    }
    int size(const string &s){
        int cur = 0;
        for(auto c:s){
            if(tree[cur][c - 'a']==-1)return 0;
            cur = tree[cur][c - 'a'];
        }
        return tree[cur].sz;
    }
    int size(int node_num){
        return (node_num==-1?0:tree[node_num].sz);
    }
    int search(string &s){
        int cur=0,ans=0;
        array<int,26> t;
        t.fill(0);
        for(auto c:s)t[c-'a']++;
        for(auto c:s){
            for(int j=0;j<26;j++){
                if(t[j])ans+=size(tree[cur][j]);
            }
            if(tree[cur][c-'a']==-1)return ans;
            cur=tree[cur][c-'a'];
            t[c-'a']--;
        }
        return ans;
    }
};

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    int n, m;
    cin >> n >> m;
    Trie trie;
    for(int i = 0; i < n; i++){
        string s;
        cin >> s;
        trie.insert(s);
    }
    string ans;
    int th = n - m; // th以下ならOK
    auto dfs = [&](auto dfs, int v, int ofs) -> void {
        if(ofs + trie[v].sz <= th){
            cout << "Yes\n";
            cout << ans << '\n';
            exit(0);
            return;
        }
        ofs += trie[v].hik;
        if(ofs > th) return;
        for(int i = 0; i < 26; i++){
            if(trie[v][i] == -1) continue;
            ans += 'a' + i;
            dfs(dfs, trie[v][i], ofs);
            ans.pop_back();
        }
    };
    dfs(dfs, 0, 0);
    cout << "No\n";
}