func main()
	var n: int :: cui@inputInt()
	var p: []int :: #[n]int
	var a: []int :: #[n]int
	for i(0, n - 1)
		do p[i] :: cui@inputInt()
		do a[i] :: cui@inputInt()
	end for
	
	; i本目, j = 0 => p本を選んだ。j=1 => a本を選んだ。
	var dp: [][]int :: #[n, 2]int
	do dp[0][0] :: p[0]
	do dp[0][1] :: a[0]
	for i(1, n - 1)
		do dp[i][0] :: [dp[i - 1][0], dp[i - 1][1] + p[i]].max() + p[i]
		do dp[i][1] :: [dp[i - 1][0], dp[i - 1][1] + a[i]].max() + a[i]
	end for
	var ans: int :: dp[n - 1][0]
	do cui@print("\{ans}\n")
end func