A, B, C, D = map(int,input().split())
All = (B-A+1)*(D-C+1)
Red = set([])
Black = set([])
Product = []
for i in range(A, B+1):
    Red.add(i)
for j in range(C, D+1):
    Black.add(j)
Product = Red&Black
print(All - len(Product))