N, M = gets.split.map(&:to_i)
s = []
c = []
str = Hash.new
M.times do |i|
  s[i], c[i] = gets.split.map(&:to_i)
  if str[s[i]-1].nil?
    str[s[i]-1] = []
  end
  str[s[i]-1] << c[i]
end

puts "#{str}"

ans = []
ans = ans.fill('0',0..N-1)
if N>=2
  ans[0] = '1'
end

str.each do |key, val|
  if key==0 and val.first==0 and N>=2
    puts -1
    exit
  else
    val.uniq!
    if val.size >= 2
      puts -1
      exit
    else
      ans[key] = val.first.to_s
    end
  end
end
puts "#{ans.inject(:+)}"