def main():
    n, m = list(map(int, input().split()))
    S = [input() for _ in range(n)]
    i = 0
    j = 0
    Ans = []
    while m and i < 26:
        #print(Ans, m, S)
        c = chr(97+i)
        cnt = 0
        cnt0 = 0
        for s in S:
            if s[j] == c and len(s)-1 == j:
                cnt += 1
            if s[j] != c:
                cnt0 += 1
        if m <= cnt0:
            return "".join(Ans) + c
        if len(S)-cnt < m:
            T = []
            for s in S:
                if s[j] != c:
                    T.append(s)
                else:
                    m -= 1
            i += 1
            if m <= 0 and i < 26:
                return "".join(Ans) + chr(97+i)
            S = T
        else:
            Ans.append(c)
            T = []
            for s in S:
                if s[j] == c and len(s)-1 == j:
                    pass
                elif s[j] != c:
                    m -= 1
                else:
                    T.append(s)
            S = T
            i = 0
            j += 1
    return ""

ans = main()
if ans == "":
    print("No")
else:
    print("Yes")
    print(ans)