Given two different integers $a$ and $b$. Find such integer $k$ that $|a - k| = |b - k|$. Note: $|x|$ means the absolute value of $x$. \InputFile Two integers $a$ and $b~(-10^9 \le a, b \le 10^9, a \ne b)$. \OutputFile If the value of $k$ doesn't exist, print "\textbf{-}". Otherwise print $k$.