# test = '1 1 1 1'
a, b, c, d = map(int, input().split())
# a, b, c, d = map(int, test.split())
x = b - a + 1
y = d - c + 1
dup = 0

if b < c:
    dup = 0
elif c < a:
    dup = 0
else:
    if a <= c and c < b:
        # print('a')
        dup = b - c + 1
    elif a <= c and b <= d:
        # print('b')
        dup = d - a + 1
    elif c <= b and b < d:
        # print('c')
        dup = b - c + 1
    elif c <= a and b <= d:
        # print('d')
        dup = d - c + 1

print(x * y - dup)