def sum_each_to_other(list) 
    return list.first if list.count <= 1 
    sum_list=[]
    for i in (0..list.count-2) do
        sum_list.push list[i] + list[i+1]
    end
    sum_each_to_other(sum_list)
end
a = gets.to_i
puts sum_each_to_other(gets.split.map(&:to_i))