N, M = map(int, input().split())
S = [input() for i in range(N)]

cnt = [0] * 26
for i in range(N):
	cnt[ord(S[i][0]) - ord('a')] += 1

for c in range(26):
	if cnt[c] + M > N:
		continue
	print("Yes")
	print(chr(ord('a') + c))
	break
else:
	print("No")