first = input().split()
A = int(first[0])
B = int(first[1])
C = int(first[2])
D = int(first[3])
ABrange = B - A + 1
CDrange = D - C + 1
ans = ABrange*CDrange
if ((A>C)and(D<B))or((A>C)and(D<B)):
    ans = 0
elif (B <= C) or  (D <= A):
    ans = ABrange*CDrange
elif B > D :#
    double = D - A  +1
    n = (ABrange - double) * CDrange
    m = double * (CDrange - double)
    ans = n+ m
elif B < D :
    double = B - C  +1
    n = (CDrange - double) * ABrange
    m = double * (ABrange - double)
    ans = n+ m
print(ans)