dp = [0, 0]
gets.to_i.times do
	a, b = gets.split.map &:to_i
	dp = [
		[dp[0] + a, -2[dp[1]] * (dp[1] + a * 2)].max,
		[dp[0] + b, -2[dp[1]] * (dp[1] + b * 2)].max
	]
end
puts dp.max