string = input()

def scan_pairs(to_scan):
    pairs = list()

    for i in range(len(to_scan)):
        if i < len(to_scan)-1:
            if ' ' not in to_scan[i] and ' ' not in to_scan[i+1]:
                pairs.append(to_scan[i]+to_scan[i+1])

    return pairs

def most_common(to_scan):
    pairs = scan_pairs(to_scan)
    count = 0
    m_count = 0
    the_pair = ""
    for i in range(len(pairs)):
        current_pair = pairs[i]
        for j in range(len(pairs)):
            if current_pair == pairs[j]:
                count += 1

        if count >= m_count:
            m_count = count
            the_pair = current_pair

        count = 0
    return the_pair

print(most_common(string))