A, B, C, D = map(int, input().split())

AB_l =[]
CD_l =[]

for i in range(A,B+1):
    AB_l.append(i)

for j in range(C, D + 1):
    CD_l.append(j)

ans_l = set(AB_l + CD_l)

print((B-A +1) * (D-C + 1) - len(AB_l + CD_l) + len(ans_l))