# after YouTube explanation
n = gets.strip.to_i
a = gets.strip.split.map{|e| e.to_i - 1}
dp = Array.new(n+1){Array.new(n+1,-1_000_000_000)}
upd = ->(i,j,x) {
  dp[i][j] = x if dp[i][j] < x
  dp[j][i] = x if dp[j][i] < x
  dp[i][n] = x if dp[i][n] < x
  dp[n][i] = x if dp[n][i] < x
  dp[n][j] = x if dp[n][j] < x
  dp[j][n] = x if dp[j][n] < x
  dp[n][n] = x if dp[n][n] < x
}
upd.call(a[0],a[1],0)

ax = 0
2.step(n*3-3, 3) do |ai|
  x = a[ai]
  y = a[ai+1]
  z = a[ai+2]
  if x==y && y==z
    ax += 1
    next
  end
  q = []
  3.times do
    (n+1).times do |ai| # dp[a][?]
      now = dp[ai][n]
      now = dp[ai][y]+1 if y == z && dp[ai][y]+1 > now
      q.push([ai,x,now])
    end
    now = [dp[n][n], dp[z][z]+1].max  # dp[x][y]
    q.push([x,y,now])
    x,y,z = y,z,x
  end
  q.each do |x,y,now|
    upd.call(x,y,now)
  end
end
ans = dp[n][n]
l = a[-1]
ans = dp[l][l]+1 if dp[l][l]+1> ans
ans += ax
p ans
