def up_int(num):
    if int(num) == num:
        return int(num)
    else:
        return int(num)+1

n = int(input())
data = list(map(int, input().split()))
dp  = [0] * (n+1)  # 最后一个位置最小为0
for i in range(n-1, -1, -1):
    dp[i] = up_int((dp[i+1]+data[i])/2)
print(dp[0])