Problem:
Convex Hull

Find the convex hull of a given set of points P. In other words, find the smallest convex polygon containing all the points of P. Here, in a convex polygon, all interior angles are less than or equal to 180 degrees.

Please note that you should find all the points of P on both corner and boundary of the convex polygon.

Input

n
x1 y1
x2 y2
:
xn yn

The first integer n is the number of points in P. The coordinate of the i-th point pi is given by two integers xi and yi.

Output

In the first line, print the number of points on the corner/boundary of the convex polygon. In the following lines, print x y coordinates of the set of points.
The coordinates should be given in the order of counter-clockwise visit of them starting from the point in P with the minimum y-coordinate, or the leftmost such point in case of a tie.

Constraints

3 ≤ n ≤ 100000

-10000 ≤ xi, yi ≤ 10000

No point in the P will occur more than once.

Sample Input 1

7
2 1
0 0
1 2
2 2
4 2
1 3
3 3

Sample Output 1

5
0 0
2 1
4 2
3 3
1 3

Sample Input 2

4
0 0
2 2
0 2
0 1

Sample Output 2

4
0 0
2 2
0 2
0 1

Solution in Ruby (judge verdict: Wrong Answer)

n = gets.to_i

list = []
s = [1.0/0.0, 1.0/0.0]
n.times{|i|
  list[i] = gets.split.map(&:to_i)
  if list[i][1] < s[1] || (list[i][1] == s[1] && list[i][0] < s[0])
    s = [list[i][0],list[i][1]]
  end
}

list.sort! {|a, b| a[1] == b[1] ? a[0] <=> b[0] : a[1] <=> b[1]}
a = list[0]
used = []
loop do
  used << a
  b = list[0]
  1.upto(n-1){|i|
    c = list[i]
    if b == a
      b = c
    else
      v = (b[0]-a[0])*(c[1]-a[1])-(b[1]-a[1])*(c[0]-a[0])
      if v > 0 || (v == 0 && (c[0]-a[0])**2+(c[1]-a[1])**2 > (b[0]-a[0])**2+(b[1]-a[1])**2)
        b = c
      end
    end
  }
  a = b
  break if a == used[0]
end

p used.reverse

while s != used[0]
  used.push(used.shift)
end
puts used.size
used.each{|pt|
  puts "#{pt[0]} #{pt[1]}"
}