# ユークリッドの互除法で最大公約数を求める関数
def gcd(a, b):
    if b == 0:
        return a
    return gcd(b, a % b)

def question15(p1, p2):
    return gcd(abs(p1[0] - p2[0]), abs(p1[1] - p2[1])) - 1

x1 =  -2
y1 =  -9
x2 =  6
y2 =  7
question15([x1, y1], [x2, y2])

7

x1 =  -42
y1 =  -65
x2 =  62
y2 =  -91
question15([x1, y1], [x2, y2])

25

x1 =  908
y1 =  -307
x2 =  -86
y2 =  -679
question15([x1, y1], [x2, y2])

1

x1 =  -6326
y1 =  3211
x2 =  7048
y2 =  5822
question15([x1, y1], [x2, y2])

0

x1 =  -9675
y1 =  -2803
x2 =  3828
y2 =  -6349
question15([x1, y1], [x2, y2])

2