n,m=map(int,input().split())
a=[0]*n
v=[0]*n
f=0
for _ in range(m):
	s,c=map(int,input().split())
	if(v[s-1]==0 or a[s-1]==c):
		v[s-1]=1
		a[s-1]=c
	else:
		f=1
s1='a'
for x in a:
	s1+=str(x)
s1=s1[1:]
if f==1 or len(str(int(s1)))!=n:
	print(-1)
else:
	print(s1)
