# coding utf-8

s = str(input())
s = s.split(" ")

a = int(s[0])
b = int(s[1])
c = int(s[2])
d = int(s[3])

if a < c:
    e = b - c + 1
    if d < b:
        e -= (b - d)
else:
    e = d - a + 1
    if b < d:
        e -= (d - b)
    
if e <= 0:
    e = 0

ans = (b - a + 1) * (d - c + 1) - e

print(ans)