# No.799 赤黒かーどげぇむ
a, b, c, d = [int(i) for i in input().split()]

cards_red = list(range(a, b + 1))
cards_black = list(range(c, d + 1))

cases = 0

for i in cards_red:
    for j in cards_black:
        if i != j:
            cases += 1

print(cases)