Codeforces #341 Div2

cf341-div2-a.rb

n = gets.chomp.to_i
a = gets.chomp.split(" ").map(&:to_i)

aoddmin = a.select{|x| x.odd?}.min

asum = a.inject(:+)
if asum.even? then
  puts asum
else
  puts asum - aoddmin
end

cf341-div2-b.rb

n = gets.chomp.to_i
coords1 = Hash.new(0)
coords2 = Hash.new(0)
n.times do
  coord = gets.chomp.split(" ").map(&:to_i)
  coords1[coord[1] - coord[0]] += 1
  coords2[coord[1] + coord[0]] += 1
end

def comb(n, k)
  return ((n - k + 1)..n).inject(:*) / (1..k).inject(:*)
end

res = 0
coords1.values.each do |v|
  res += comb(v, 2)
end
coords2.values.each do |v|
  res += comb(v, 2)
end

puts res

cf341-div2-c.rb

n, p = gets.chomp.split(" ").map(&:to_i)
lr = []
n.times do 
  lr << gets.chomp.split(" ").map(&:to_i)
end
res = 0
n.times do |i|
  l1, r1 = lr[i]
  l2, r2 = lr[i - 1]
  a1 = r1 / p - (l1 - 1) / p
  a2 = r2 / p - (l2 - 1) / p
  res += 2000 * (1 - (r1 - l1 + 1 - a1) / (r1 - l1 + 1).to_f * (r2 - l2 + 1 - a2) / (r2 - l2 + 1).to_f)
end
puts res