from operator import mul
from functools import reduce

def cmb(n,r):
    r = min(n-r,r)
    if r == 0: return 1
    over = reduce(mul, range(n, n - r, -1))
    under = reduce(mul, range(1,r + 1))
    return over // under

def max(a,b):
    return a if a >= b else b

input_list = list(map(int,input().split()))
a = input_list[0]
b = input_list[1]
c = input_list[2]
d = input_list[3]

# a~b, c~d中の、重複を許した数字の個数
num_overlap = (b - a + 1) * (d - c + 1)

# # num_overlapから2個とる組み合わせの数
# comb_overlap = cmb(num_overlap, 2)

# print('comb_overlap: {}'.format(comb_overlap))

# 被っている範囲の組み合わせの数
range_overlap = 0
if d < a or b < c:
#     print('ダブり無し')
    range_overlap = 0
else:
    if d < b:
#         print('pass1')
        range_overlap = d - max(a, c) + 1
    else:
        if a < c:
#             print('pass2')
            range_overlap = b - c + 1
        else:
#             print('pass3')
            range_overlap = b - a + 1

res = num_overlap - range_overlap

print(res)