n,m=map(int,input().split())
if m==0:
    print(-1)
else:
        
    tf=1
    ans=[set()for i in range(n)]
    for i in range(m):
        s,c=map(int,input().split())    
        if s==1 and c==0:
            tf=0
        ans[s-1].add(c)
    if not tf:
        print(-1)
    else:
            
        f=1
        an=''
        #print(ans)
        ii=0
        for i in ans:
            if ii==0 and len(i)==0:
                an+='1'
                ii+=1
                continue
            ii+=1    
            if len(i)>1:
                f=0
                break
            if len(i)==0 and ii!=0:
                an+='p'
            else:
                an+=str(i.pop())
            
        if not f:
            print(-1)
        else:
            an=an.replace('p','0')
            if an[0]=='0':
                print(-1)
            else:    
                print(an)
        
            
            
            
            
        