#!/bin/bash

read a b c;
c=${b}-${a}
d=${c}-${b}
if [ $(($b - $a)) -eq $(($c - $b)) ];
then 
    echo YES;
else 
    echo NO;
fi
# if [ ${x} -lt 1200 ]
# then 
#     echo ABC
# else
#     echo ARC
# fi
exit 0