n,m=map(int,input().split())
sc= [list(map(int,input().split())) for i in range(m)]
ans = -1
for j in range(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)