def censurera(ord1):
    nytt_ord = ""
    for k in range(len(ord1)):
        if(ord1[k] == 'b'):
            nytt_ord = nytt_ord + 'X'
        else:
            nytt_ord = nytt_ord + ord1[k]
    return nytt_ord

censurerat_ord = censurera("aaaabbbbcccc")
print(censurerat_ord)