line = input().split(" ")
min1 = int(line[0])
max1 = int(line[1])
min2 = int(line[2])
max2 = int(line[3])

list1 = list(range(min1, max1 + 1))
list2 = list(range(min2, max2 + 1))
cnt = 0

for num1 in list1:
    for num2 in list2:
        if (num1 != num2):
            cnt += 1

print(cnt)