a, b, c, d = map(int, input().split())

l1 = [i for i in range(a, b+1)]
l2 = [i for i in range(c, d+1)]

l3 = []
for i in l1:
    for j in l2:
        if i != j:
            l3.append([i ,j])

print(len(l3))