n,m=map(int,input().split())
 
a=["0"]*n
 
for i in range(m):
    s,c=map(int,input().split())
    if a[s-1]=="0":
        a[s-1]=c
    elif a[s-1]!=c:
        print(-1)
        exit()

if n>1 and a[0]==0:
    print(-1)
    exit()

if n>1 and a[0]=="0":
    a[0]=1

print(*a,sep="")