N = gets.to_i
Q = N.times.map { gets.split.map(&:to_i) }

dp = Array.new(N + 1) { Array.new(2, 0) }
dp[0][0] = Q[0][0]
dp[0][1] = Q[0][1]

(1...N).each do |i|
  q, a = Q[i]
  _, ba = Q[i - 1]

  dp[i][0] = [dp[i - 1][0] + q, dp[i - 1][1] + 2 * q].max
  dp[i][1] = [dp[i - 1][0] + a, dp[i - 1][1] + 2 * a].max
end

puts dp[N - 1].max