N = int(input())
sushi = [ int(i) for i in input().split() ]

if N == 1:
    print(sushi[0])
    exit()

result = 0
i = 0

while i <= N-1:
    if i == N-1 or sushi[i] >= sushi[i+1]:
        high = i
    else:
        high = i + 1
        
    result += sushi[high]
    i = high + 2
    
print(result)