n,m = map(int,input().split())
d = {}
for _ in range(m):
  s,c = map(int,input().split())
  if s in d:
    if d[s] == c:
      continue
    else:
      print(-1)
      exit()
  else:
    d[s] = c

ans = ''
for i in range(n):
  if i+1 in d:
    ans += str(d[i+1])
  else:
    ans += '1'
print(ans)