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 r[int(int(p[0])-1)] != "0" and r[int(int(p[0])-1)] != p[1]:
    ans = "-1"
  r[int(int(p[0])-1)] = p[1]
  
res = ""
if not (ans == "-1"):
  for i in range(len(r)):
    res = res + r[i]
  ans = res
  ans = ans.strip("0")
  if ans == "":
    ans = "-1"

print(ans)