n, m = map(int, input().split())
ans = [0]*n

for _ in range(m):
  s, c = map(int, input().split())
  if ans[s-1] == 0:
    ans[s-1] = c
  elif ans[s-1] >= c:
    ans[s-1] = c
 
if ans[0] != 0:
  L=[str(a) for a in ans]
  print("".join(L))
else:
  print(-1)