N,M = map(int,input().split())
sc=[list(map(int,input().split()))for _ in range(M)]

ans = -1
for i in range(1000):
  s = str(i)
  if len(s) == N:
    flg = 0
    for a,b in sc:
      if int(s[a-1]) != b:
        flg = 1
    if flg == 0:
      ans = i
      break
print(ans)