o = input().split()
n = int(o[0])
m = int(o[1])
 
ans = ""
 
r = []
for i in range(n):
  r.append(0)
for i in range(m):
  p = input().split()
  if [int(p[0])-1] != 0:
    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
    ans = ans.strip("0")
    if ans == "":
      ans = "-1"

print(ans)