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:
		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
print(ans)