已知空间四边形\(OABC\),点\(M,N\)分别为\(OA,BC\)的中点,且\( \overrightarrow{OA}= \overset{→}{a}, \overrightarrow{OB}= \overset{→}{b}, \overrightarrow{OC}= \overset{→}{c} \),用\(\vec{a}\),\(\vec{b}\),\(\vec{c}\)表示\( \overrightarrow{MN} \),则\( \overrightarrow{MN} =\)_____________。