import pypyjit
pypyjit.set_param("max_unroll_recursion=-1")
import sys
sys.setrecursionlimit(10**5)
"""RollingHash

・つかい方
    rh = RollingHash(S): インスタンス生成
    rh.get(l, r): hash(S[l, r))

・めも
    1つ目のインスタンス生成で係数cfを決めてしまうから、
    たくさんインスタンスつくっても整合性がとれてる。

・参考URL
    https://qiita.com/keymoon/items/11fac5627672a6d6a9f6
"""

import random


class RollingHash:
    def __init__(self, S: str):
        le = len(S)
        if RConst.cf == 0:
            RConst.cf = random.randrange(1 << 31, RConst.Mod)
        if len(RConst.rui_cf) <= 5*10**5+100:
            RConst.make_rui_cf(5*10**5+100)
        self.hash_arr = [1]
        for el in S:
            self.hash_arr.append(RConst.calc_mod(
                RConst.mul(self.hash_arr[-1], RConst.cf) + ord(el)))

    def append(self, s:str):
        self.hash_arr.append(RConst.calc_mod(
            RConst.mul(self.hash_arr[-1], RConst.cf) + ord(s)))

    def pop(self):
        self.hash_arr.pop()

    def get(self, l: int, r: int) -> int:
        "hash(S[l..r))"
        return RConst.sub(self.hash_arr[r],
                          RConst.mul(self.hash_arr[l], RConst.rui_cf[r - l]))


class RConst:
    Mask30 = (1 << 30) - 1
    Mask31 = (1 << 31) - 1
    Mod = (1 << 61) - 1
    cf = 0
    rui_cf = [1]

    @staticmethod
    def calc_mod(x: int) -> int:
        xu, xd = x >> 61, x & RConst.Mod
        ret = xu + xd
        if RConst.Mod <= ret:
            ret -= RConst.Mod
        return ret

    @staticmethod
    def sub(a: int, b: int) -> int:
        if a < b:
            return a + RConst.Mod - b
        return a - b

    @staticmethod
    def mul(a: int, b: int) -> int:
        au, ad = a >> 31, a & RConst.Mask31
        bu, bd = b >> 31, b & RConst.Mask31
        mid = ad * bu + au * bd
        midu, midd = mid >> 30, mid & RConst.Mask30
        return RConst.calc_mod(((au * bu) << 1) + midu + (midd << 31) + ad * bd)

    @staticmethod
    def make_rui_cf(x: int):
        l = len(RConst.rui_cf)
        for _ in range(x - l + 1):
            RConst.rui_cf.append(RConst.mul(RConst.cf, RConst.rui_cf[-1]))

from collections import defaultdict
N,M = list(map(int,input().split()))
S = [input() for _ in range(N)]

d = defaultdict(int)
end = defaultdict(int)
for i in range(N):
    rh = RollingHash(S[i])
    for j in range(len(S[i])):
        d[rh.get(0,j+1)] += 1
    end[rh.get(0,len(S[i]))] += 1

ans = []
end_count = 0
now_RH = RollingHash("")
def f():
    global end_count
    for i in range(26):
        c = chr(ord("a") + i)
        new = "".join(ans + [c])
        now_RH.append(new)
        hash = now_RH.get(0,len(ans)+1)
        end_count += end[hash]

        if(end_count < N - M + 1):
            if(d[hash] + end_count - end[hash] >= N - M + 1):
                ans.append(c)
                f()
                ans.pop()
            else:
                print("Yes")
                print("".join(ans + [c]))
                exit()

        end_count -= end[hash]
        now_RH.pop()
        
f()
print("No")