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

sets = []
for i in range(M):
  s,c = input().split()
  sets.append((s,c))
  
for n in range(10**(N-1), 10**N-1):
  for s,c in sets:
    if not str(n)[int(s)-1] == c:
      break
  else:
    print(n)
    break
else:
  print(-1)