n,m=map(int,input().split())
if m==0:
  if n ==1:
    print(0)
  else:
    print(10**(n-1))
else:
  sc= [list(map(int,input().split())) for i in range(m)]
  ans = -1
  for j in range(10**(n-1),10**n):
    k=str(j)
    s=[[0 for i in range(2)] for i in range(len(k))]
    for l in range(len(k)):
      s[l][0]=l+1
      s[l][1]=int(k[l])
      if all(elm in s for elm in sc) == 1:
        ans=j
        break
    if ans==j:
      break
  print(ans)