四位加满了会怎样

运行下面这段程序，填写它打印出来的结果。

def half_adder(a, b):
    return a ^ b, a & b

def full_adder(a, b, cin):
    s1, c1 = half_adder(a, b)
    s2, c2 = half_adder(s1, cin)
    return s2, c1 | c2

def add4(x, y):
    c = 0
    bits = []
    for i in range(4):
        s, c = full_adder((x >> i) & 1, (y >> i) & 1, c)
        bits.append(s)
    return "".join(str(v) for v in reversed(bits)) + "/" + str(c)

print(add4(9, 7))