A, B, C, D = map(int, input().split())
ab = (B - A + 1)
cd = (D - C + 1)
ans = ab*cd
if A == B and B == C and C == D:
    ans = 0
elif A <= C and B >= C and B <= D:
    ans -= (B - C + 1)
elif A >= C and A <= D and B >= D:
    ans -= (D - A + 1)
elif A < C and B > D:
    ans -= cd
elif A > C and B < D:
    ans -= ab
print(ans)