Weierstraß Product Inequality

If $0\leq a, b, c, d \leq 1$, then

$$(1-a)(1-b)(1-c)(1-d)+a+b+c+d\geq 1.$$

References

Honsberger, R. Mathematical Gems III. Washington, DC: Math. Assoc. Amer., pp. 244-245, 1985.