o = input().split()
n = int(o[0])
m = int(o[1])
ans = ""
r = [0 for i in range(n)]
for i in range(m):
  p = input().split()
  if r[int(p[0])-1] != 0 and r[int(p[0])-1] != p[1]:
    ans = "-1"
    break
  r[int(p[0])-1] = p[1]
res = ""
if ans == "-1":
  pass
else:
  for i in range(len(r)):
    res = res + str(r[i])
    ans = res
    if len(ans) != len(ans.strip("0")):
      ans = "-1"
print(int(ans))