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