from math import pow, log
def convert(n,a,b):
    n=str(n)
    " n est un str, a et b des entiers "
    chiffres = ['0','1','2','3','4','5','6','7','8','9','a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z']
    nb_decimal = 0
    i = len(n)-1
    for x in n:
        nb_decimal += chiffres.index(x)*int(pow(a,i))
        i= i-1
    nbr_chars = int(log(nb_decimal)/log(b)+1)
    res = ""
    for i in range(0, nbr_chars):
        x = int(nb_decimal / pow(b,nbr_chars-1-i))
        nb_decimal -= int(x * pow(b,nbr_chars-1-i))
        res = res+chiffres[x]
    return res