str1=input()
l = str1[0:len(str1)-2].split(' ')
max=''
if 'a' not in str1:
    print('NO')
else:
    for i in l:
        if 'a' in i:
            if len(i)>len(max):
                max=i
    print(max)

/**************************************************************
	Problem: 1111
	User: admin
	Language: Python
	Result: Accepted
	Time:153 ms
	Memory:34480 kb
****************************************************************/