N, M = map(int, input().split())
sc = [list(map(int, input().split())) for i in range(M)]
sc.sort()
pin = 0
if N == 1:
  tmp = sc[0][1]
  for i in range(M):
    if tmp != sc[i][1]:
      print(-1)
      pin = 1
      break
  if pin == 0:
    print(tmp)
else:
  pin = 0
  ans_lis = [0] * N
  tmp_s = sc[0][0]
  tmp_c = sc[0][1]
  for i in range(M):
    if sc[i][0] == 1 and sc[i][1] == 0:
      print(-1)
      pin = 1
      break
    if tmp_s == sc[i][0]:
      if tmp_c != sc[i][1]:
        print(-1)
        pin = 1
        break
    else:
      ans_lis[tmp_s -1] = tmp_c
      tmp_s = sc[i][0]
      tmp_c = sc[i][1]
  if pin == 0:
    ans_lis[tmp_s -1] = tmp_c
    if ans_lis[0] == 0:
      ans_lis[0] = 1
    lis = map(str, ans_lis)
    mojiretsu = ''
    for x in lis:
      mojiretsu += x
    print(mojiretsu)
      
      
