我有一个程序需要两个英制长度（英里，码，英尺和英寸）并输出它们的总和和差异。我的问题是，当差异为负时，输出是不正确的。
def to_inches(miles, yards, feet, inches):
return inches + (feet * 12) + (yards * 36) + (miles * 63360)
def from_inches(inches):
miles = inches // 63360
inches %= 63360
yards = inches // 36
inches %= 36
feet = inches // 12
inches %= 12
return (miles, yards, feet, inches)
units1 = [int(n) for n in input().split(' ')]
units2 = [int(n) for n in input().split(' ')]
m_sum = from_inches(to_inches(*units1) + to_inches(*units2))
m_diff = from_inches(to_inches(*units1) - to_inches(*units2))
print(' '.join(str(n) for n in m_sum))
print(' '.join(str(n) for n in m_diff))
输入：
2 850 1 7
1 930 1 4
输出：
4 21 0 11
0 1680 1 3
输入：
0 1 0 0
1 0 0 0
预期产出：
1 1 0 0
-0 1759 0 0
实际输出：
1 1 0 0
-1 1 0 0