def index(list=[])
    sum_left = 0
    sum_right = list.inject(0,:+)
    list.each_with_index do |value, index|
        sum_right -= value
        return index if sum_right == sum_left
        sum_left += value
    end
    -1
end

list = [1,2,3,4,3,2,1]
print index(list)