n = int(input())

odd_sum= 0
odd_min= -90000000000
odd_max = -9000000000
even_sum= 0
even_min=-9000000000
even_max = -900000000

for number in range(1, n+1):
    current_num= float(input())
    if number%2== 0:
        even_sum+= current_num
        if current_num > even_max:
            current_num = even_max
        elif current_num < even_min:
            current_num = even_min
    else:
        odd_sum+= current_num
        if current_num > odd_max:
            current_num= odd_max
        elif current_num < odd_min:
            current_num = odd_min