def rle(s):
    en, c, cc = "", s[0], 0
    for i in s:
        if i == c: cc += 1
        else:
            en += (c if (cc == 1) else c + str(cc))
            c, cc = i, 1
    en += (c if (cc == 1) else c + str(cc))
    return en

print(rle("abbbcccd"))