N,M = map(int, input().split())
if N == 1:
  candidate = [-1]
  if M == 0:
    print(0)
  else:
    s_temp,c_temp = map(int, input().split())
    print(c_temp)
else:
  candidate = [-1]*N
  flag = False
  for i in range(M):
    s_temp,c_temp = map(int, input().split())
    if candidate[s_temp-1] == -1:
      candidate[s_temp-1] = c_temp
    elif candidate[s_temp-1] == c_temp:
      pass
    else:
      flag = True
      break
  if flag == True or (candidate[0] == 0 and N != 1):
    print(-1)
  else:
    if candidate[0] == -1:
      candidate[0] = 1
    output_list = [0 if i == -1 else i for i in candidate]
    print(int(''.join(map(str, output_list))))