package main

import "fmt"

func main() {
	var n int
	fmt.Scan(&n)

	nums := make([]int, n)
	for i := 0; i < n; i++ {
		fmt.Scan(&nums[i])
	}

	fmt.Println(maxSum(nums))
}

func maxSum(nums []int) int {
	if len(nums) == 0 {
		return 0
	}
	if len(nums) == 1 {
		return nums[0]
	}

	dp := make([]int, len(nums))
	dp[0] = nums[0]
	dp[1] = max(nums[0], nums[1])

	for i := 2; i < len(nums); i++ {
		dp[i] = max(dp[i-1], dp[i-2]+nums[i])
	}

	return dp[len(nums)-1]
}

func max(a, b int) int {
	if a > b {
		return a
	}
	return b
}