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Competitions

Easy problem for Dino

The math teacher asks Dino a very simple question: Given two different integers a and b. Find such integer k that |a - k| = |b - k|. Warning: |x| means absolute value.

Input

Two integers a and b (-109a, b109, ab).

Output

If the value of k doesn't exist, print - , otherwise print k.

Time limit 1 second
Memory limit 128 MiB
Input example #1
10 5
Output example #1
-
Input example #2
6 2
Output example #2
4