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]=min(ans[s-1],c)
  else:
        ans[s-1]=c
if ans[0]==0:
      print(-1)
      exit()
for i in ans:
  print(i,end="")