def convert_base(N, b):
    """
    convert_base(integer, integer) -> string
 
    Return the textual representation of N (decimal) in base 2 <= b <= 36.
    """
    assert 2<=b<=36

    if N==0:
        return "0"
    
    result = ""
    while N!=0:
        digit = N%b
        N = N//b
        #result = str(digit) + result
        result = "0123456789abcdefghijklmnopqrstuvwxyz"[digit] + result

    return result