l = [[int(i.split(' ')[0]), int(i.split(' ')[1])] for i in open('paint.in', 'r')]

l = sorted(l, key=lambda x: x[0])

a = l[0][0]
b = l[0][1]
c = l[1][0]
d = l[1][1]

if c <= b:
    out = max(d, b) - a

else:
    out = (b - a) + (d - c)

open('paint.out', 'w').write(str(out))