N,M=map(int,input().split())
s=[0]*M
c=[0]*M
a=0
for i in range(M):
	s[i],c[i]=map(int,input().split())
ans=-1
for j in range(1,1000):
	value=str(j)
	if len(value)==N:
		if max(s)<=N:
			for S,C in zip(s,c):
				if int(value[S-1])==C:
					a+=1
			if a==M:
				ans=int(j)
				break
			else:
				a=0
		else:
			continue
	else:
		continue
print(ans)