Submission #3232087

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Source Code Expand

N,M = gets.split.map(&:to_i)
xs = Array.new(1e5+1,0)
gets.split.each{ |a|
  xs[a.to_i] += 1
}
pairs = Array.new(M, 0)
singles = Array.new(M, 0)
xs.each_with_index { |n,i|
  pairs[i%M] += n/2
  singles[i%M] += n%2
}
#p pairs, singles

ans = 0
ans += pairs[0] + singles[0]/2
if M%2 == 0
  ans += pairs[M/2] + singles[M/2]/2
end
(1...(M+1)/2).each{ |k|
  #p ans
  le,ma = k,M-k
  if singles[k] > singles[M-k]
    le,ma = ma,le
  end
  ans += singles[le]
  ans += pairs[ma]
  ans += pairs[le]
  ans += [pairs[le], (singles[ma] - singles[le])/2].min
}
p ans

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