n, m = gets.chomp.split(' ').map(&:to_i)
ans = Array.new(n, nil)

m.times do
  s, c = gets.chomp.split(' ').map(&:to_i)
  if !ans[s-1].nil? && ans[s-1] != c 
    print(-1)
    exit
  else
    ans[s-1] = c
  end
end

ans[0] = 1 if ans[0].nil?
# print(ans)
print(ans.map{|e| e ? e : 0}.join.to_i)
