import fire

def r(s):
    if len(s) == 0:
        return ""

    viewed = {s[0]}
    longuest = s[0]
    a = 0
    b = 1

    while b < len(s):
        if s[b] in viewed:
            viewed.remove(s[a])
            a += 1

        else:
            if (b - a + 1) > len(longuest):
                longuest = s[a:b+1]
            viewed.add(s[b])
            b += 1

    return longuest

if __name__ == '__main__':
    fire.Fire(r)