l =[int(i)for i in  input().split()]
n = l[1]-l[0]+1
m = l[3]-l[2]+1
ans = m*n
A=set([i for i in range(l[0],l[1]+1)])
B=set([i for i in range(l[2],l[3]+1)])
C =A & B
ans -= len(list(C))
print(ans)