Definition

from: www.dictionary.com

de·pres·sion (d-prshn)
n.

1. The act of depressing or the state of being depressed.
2. A reduction in physiological vigor or activity.
3. A lowering in amount, degree, or position.
4. An inward displacement of a body part.
5. A hollow or sunken area.
6. The condition of feeling sad or despondent.
7. A psychotic or neurotic condition characterized by an inability to concentrate, insomnia, and feelings of extreme sadness, dejection, and hopelessness.

and

sadness (sd-nes)
n.

1. Affected or characterized by sorrow or unhappiness.
2. Expressive of sorrow or unhappiness.
3. Causing sorrow or gloom; depressing: a sad movie; sad news.
4. Deplorable; sorry: a sad state of affairs; a sad excuse.
5. Dark-hued; somber.

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therefore:

sadness + sadness + sadness + sadness + sadness = depression