Find the Exact Value of the Expression $\frac { 1 } { 2 }$

Multiple Choice

Find the exact value of the expression.
-cos (-390°)

A) $\frac { 1 } { 2 }$
B) $\frac { \sqrt { 3 } } { 2 }$
C) $- \frac { \sqrt { 3 } } { 2 }$
D) $- \frac { 1 } { 2 }$