n = gets.to_i
x = gets.split.map(&:to_i).sort
if x.uniq.size != x.size
    puts "NO"
else
    d = x[1] - x[0]
    a = x[1]
    2.upto(n - 1) {|i|
        a += d
        if x[i] != a
            puts "NO"
            exit
        end
        }
    puts "YES"
end