first = input().split()
A = int(first[0])
B = int(first[1])
C = int(first[2])
D = int(first[3])

AB = range(A,B+1)
CD = range(C,D+1)

double = set(AB)&set(CD)
countdouble = len(double)
countAB = len(AB)
countCD = len(CD)

ans = countAB*countCD - countdouble
print(ans)