def subs(s):
    for start in range(len(s)):
        for end in range(start, (len(s))):
            yield s[start:end]

def main():
    tests = ['000', '1111', '001010101', '0001010101', '01010101',  '000000111010101', '0000001110101011', '00000011101010111111111111111']

    for b in tests:
        most = max([item for item in subs(b) if item.count('1') == item.count('0')], key=len)
        print(most)

if __name__ == '__main__':
    main()
    input()