n,m=map(int,input().split())
p=0
a=[-1 for s in range(n)]
for i in range(m):
  s,c=map(int,input().split())
  if s==1 and c==0:
    p+=1
    break
  elif a[s-1]==-1 or a[s-1]==c:
    a[s-1]=c
  else:
    p+=1
    break
  
if p==0:
  num=0
  for i,j in enumerate(reversed(a)):
    if j == -1 and i+1 ==len(a):
      j=1
    elif j == -1:
      j=0
    num += j*(10**i)
  print(num)
else:
  print(-1)