N,M=map(int,input().split())
s=[0]*M
c=[0]*M

for i in range(M):
  s[i],c[i]= map(int,input().split())
  
if M>0:
  max_s=max(s)
else:
  max_s=100
ans=-1

for j in range(1000):
  flag=True
  k=str(j)  
  if len(k)<max_s:
    pass
  else:
    if len(k)!=N:
      pass
    else:
      for l in range(M):
        if int(k[s[l]-1])!=c[l]:
          flag=False
          break
      if flag==True:
        if ans==-1:
          ans=j
        else:
          ans=min(ans,j)
        break
      
print(ans)