n = list(map(int, input().split()))

all = (n[1] - n[0] + 1)*(n[3] - n[2] + 1)

if n[0] < n[2] and n[1] > n[3]:
    excp= n[3] - n[2] + 1
elif n[0] > n[2] and n[1] < n[3]:
    excp= n[1] - n[0] + 1
elif n[0] <= n[2] and n[1] > n[2]:
    excp = n[1] - n[2] + 1
elif n[0] >= n[2] and n[3] > n[0]:
    excp = n[3] - n[0] + 1
else:
    excp = 0

print(all - excp)