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