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 (B <= C) or  (D <= A):
    ans = ABrange*CDrange
elif B > D :
    double = D - A  +1
    ans = ans - double
elif B < D :
    double = B - C  +1
    ans = ans - double
print(ans)