def alternate(s):
    comb = list(combinations(set(s), 2))
    len_max = 0
    
    for i in comb:
        txt = s
        txt = "".join(c for c in txt if c in i)        
        if re.search(r"(.)\1", txt):
            continue
        else:
            if len(txt) > len_max:
                
                len_max = len(txt)
    
    return len_max