"""
[24310: БОЯДИСВАНЕ НА ОГРАДА](https://www.acmicpc.net/problem/24310)
Tier: Bronze 3
Category: 구현
"""
def solution():
a, b, c, d = map(int, input().split())
a, b = min(a, b), max(a, b)
c, d = min(c, d), max(c, d)
if c < a:
a, b, c, d = c, d, a, b
s = (b - a + 1) + (d - c + 1)
if c < b:
s -= (min(d, b) - c + 1)
return s
if __name__ == '__main__':
print(solution())