N, M = map(int, input().split())

S, C = [], []
for i in range(M):
  s, c = map(int, input().split())
  S += [s-1]
  C += [str(c)]
  
if N == 1:
  for i in range(10):
    flag = 1
    for m in range(M):
      if i_str[S[m]] != C[m]:
        flag = 0
        break
    if flag:
      print(i)
      break
  else:
    print(-1) 
else:
  for i in range(10**(N-1), 10**N):
    flag = 1
    i_str = str(i)
    for m in range(M):
      if i_str[S[m]] != C[m]:
        flag = 0
        break
    if flag:
      print(i)
      break
  else:
    print(-1)