from sys import argv

def pallindrome(str):
'''returns the longest substring pallindrome in str'''
	if str:
		index = 0
		if len(str) == 2:
			if str[0] == str[1]:
				length = 2
			else:
				length = 1
		else:
			length = 1
			for i in range(1,(len(str)+1)/2):
				count = 1
				for j in range(1,i+1):
					if str[i-j] == str[i+j]:
						count += 2
					else:
						break
				if count > length:
					length = count
					index = i - count/2
				count = 0
				for j in range(i):
					if str[i-j] == str[i+j+1]:
						count += 2
					else:
						break
				if count > length:
					length = count
					index = i - count/2 + 1
			tl = len(str) - 1
			for i in range(1,(len(str)+1)/2):
				tl -= 1
				count = 1
				for j in range(1,i+1):
					if str[tl-j] == str[tl+j]:
						count += 2
					else:
						break
				if count > length:
					length = count
					index = tl - count/2
				count = 0
				for j in range(i):
					if str[tl-j] == str[tl+j+1]:
						count += 2
					else:
						break
				if count > length:
					length = count
					index = tl - count/2 + 1
		return str[index:index+length]
	else:
		return ""

if __name__ == "__main__":
	if len(argv) == 3:
		ans = pallindrome(argv[1])
		for i in range(int(argv[2])-1):
			pallindrome(argv[1])
		print ans