Problem:
Score : 200 points

Problem Statement

A group of people played a game. All players had distinct scores, which are positive integers.

Takahashi knows N facts on the players' scores. The i-th fact is as follows: the A_i-th highest score among the players is B_i.

Find the maximum possible number of players in the game.

Constraints

1 \leq N \leq 10^5

1 \leq A_i \leq 10^9(1\leq i\leq N)

0 \leq B_i \leq 10^9(1\leq i\leq N)

If i ≠ j, A_i ≠ A_j.

There exists a possible outcome of the game that are consistent with the facts.

All input values are integers.

Inputs

Input is given from Standard Input in the following format:

N
A_1 B_1
:
A_N B_N

Outputs

Print the maximum possible number of players in the game.

Sample Input 1

3
4 7
2 9
6 2

Sample Output 1

8

The maximum possible number of players is achieved when, for example, the players have the following scores: 12,9,8,7,5,2,1,0.

Sample Input 2

5
1 10
3 6
5 2
4 4
2 8

Sample Output 2

7

Sample Input 3

2
1 1000000000
1000000000 1

Sample Output 3

1000000001

Solution in Ruby (2.3.3) (judge verdict: Accepted)

n = gets.to_i

ab = []
n.times do
  ab << gets.split.map(&:to_i)
end
ab.sort!{|a, b| a[0] <=> b[0] }

count  = 0
if ab.size == 1
  a, b = ab[0]
  if a == 1
    count = b + 1
  else
    count = b + a
  end
else
  top    = 0
  lowest = ab.max[0]
  ab.each do |a, b|
    if a == 1
      count += 1
    elsif top == 0 and a == 2
      count += 2
    elsif a == lowest
      count += b + (a - top)
    elsif a - top == 1
      count += 1
    else
      count += a - top
    end
    top = a
  end
end
puts count