a, b, c, d = map(int, input().split())
if a > c:
    a, c = c, a
    b, d = d, b
ans = (b - a + 1) * (d - c + 1)
if b < c:
    pass
elif c <= b <= d:
    ans -= b - c + 1
else:
    ans -= d - c + 1
print(ans)