n = gets.to_i
a = []
yes = 0
n.times do |i|
  a[i] = gets.to_s
if i >= 1 and a[i] != a[i-1]
yes += 1
elsif i >= 1 and a[i] == a[i-1]
  puts "NO"
  break
end
end
if yes == n - 1
puts "YES"
end