import sys
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]:
    print("-1")
    sys.exit()
  r[int(int(p[0])-1)] = p[1]

res = ""
for i in range(len(r)):
  res = res + r[i]
  ans = res
if len(ans) != len(ans.strip("0")):
  ans = "-1"
  
print(ans)