N, M = map(int, input().split())
N_list = [0] * N
N_set_list = [False] * N

for i in range(M):
  s, c = map(int, input().split())
  s -= 1
  
  if s > N -1 or (s == 0 and c == 0):
    print(-1)
    exit()

  elif N_set_list[s] and (N_list[s] != c):
    print(-1)
    exit()

  else:
    N_list[s] = c
    N_set_list[s] = True

if N_list[0] == 0 and len(N_list) != 1
  N_list[0] = 1

print(''.join(map(str, N_list)))
