task 19

with open('24 (1).txt') as f:
    a = f.readline()

a = sorted(map(lambda x: x[2], filter(lambda x: x[0] == x[1], zip(a, a[1:], a[2:]))))


def f(b, c):
    if b == c:
        return b
    if b != c:
        return b + ' '


b = ''.join(map(f, a, a[1:])).split()
b[-1] = b[-1] + 'Z'
sln = list(map(len, b))
print(b[sln.index(max(sln))][0])