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