#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)

struct Trie {
    struct Node {
        vector<int> next, accept;
        int c, common;
        Node(int c_) : c(c_), common(0) {
            next.assign(26, -1);
        }
    };
    vector<Node> nodes;
    int root;
    Trie() : root(0) {
        nodes.push_back(Node(root));
    }

    void insert(string s) {
        int id = 0;
        for (char c : s) {
            int num = c - 'a';
            int &nid = nodes[id].next[num];
            if (nid == -1) {
                nid = nodes.size();
                nodes.push_back(Node(num));
            }
            nodes[id].common++;
            id = nid;
        }
        nodes[id].common++;
        nodes[id].accept.push_back(nodes[0].common);
    }
    bool search(string s) {
        int id = 0;
        for (char c : s) {
            int num = c - 'a';
            int nid = nodes[id].next[num];
            if (nid == -1) {
                return false;
            }
            id = nid;
        }
        return nodes[id].accept.size() > 0;
    }
    string ans;
    string cur;
    void f(int id, int k) {
        if (ans.empty() && nodes[id].common <= k) {
            ans = cur;
        }
        int d = nodes[id].accept.size();
        rep(i, 26) {
            cur.push_back(i + 'a');
            if (nodes[id].next[i] != -1) {
                f(nodes[id].next[i], k - d);
            } else {
                if (ans.empty() && k - d >= 0) {
                    ans = cur;
                }
            }
            cur.pop_back();
        }
    }
};

int main() {
    cin.tie(nullptr);
    ios_base::sync_with_stdio(false);
    int n, k;
    cin >> n >> k;
    Trie t;
    rep(i, n) {
        string s;
        cin >> s;
        t.insert(s);
    }
    t.f(0, n - k);
    string ans = t.ans;
    if (ans.empty()) {
        cout << "No" << endl;
    } else {
        cout << "Yes" << endl;
        cout << ans << endl;
    }
    return 0;
}