Easy problem for Dino

Given two different integers $a$ and $b$. Find such integer $k$ that $|a-k|=|b-k|$.

Note: $|x|$ means the absolute value of $x$.

Input

Two integers $a$ and $b(-10^9\le a,b\le 10^9,a\ne b)$.

Output

If the value of $k$ doesn't exist, print "-". Otherwise print $k$.

Examples