Derivative of Gamma Function at 1

Theorem
Let $\Gamma$ denote the Gamma function.

Then:


 * $\map {\Gamma'} 1 = -\gamma$

where:
 * $\map {\Gamma'} 1$ denotes the derivative of the Gamma function evaluated at $1$
 * $\gamma$ denotes the Euler-Mascheroni constant.