# Solution for Google Kick Start 2021 round B problem B

def solve(a)
  # length of arithmetic array ending at i-th element (with 0 substitutions)
  l0 = [0] * a.size
  # length of arithmetic array ending at i-th element (with 1 substitution)
  l1 = [0] * a.size

  l0[0] = l1[0] = 1
  l0[1] = l1[1] = 2

  2.upto(a.size - 1) do |i|
    step0 = a[i - 1] - a[i - 2]

    if a[i] - a[i - 1] == step0
      l0[i] = l0[i - 1] + 1
    else
      l0[i] = 2
    end

    step1 = a[i - l1[i - 1] + 1] - a[i - l1[i - 1]]

    if a[i] == a[i - l1[i - 1]] + step1 * l1[i - 1]
      l1[i] = l1[i - 1] + 1
    else
      l1[i] = l0[i - 1] + 1
    end
  end

  return l1.max
end

t = gets.to_s.to_i
1.upto(t) do |casenum|
  n = gets.to_s.to_i
  a = gets.to_s.strip.split.map {|x| x.to_i }

  printf("Case #%d: %s\n", casenum, [solve(a), solve(a.reverse)].max)
end