import collections

N,M=map(int,input().split())
#if not(M==0):
s=[list(map(int,input().split())) for _ in range(M)]
flag=1
l=[[i+1,-1] for i in range(N)]
ns=""
#print(l)
"""
if M==0:
    for i in range(M):
        if i==0:
            l[i][1]=1
        else:
            l[i][1]=0
        ns="".join((ns,str(l[i][1])))
    print(ns)
"""
#else:
for i in range(M):
    #print(l)
    if l[s[i][0]-1][1]==-1:
        l[s[i][0]-1][1]=s[i][1]
    else:
        if not(l[s[i][0]-1][1]==s[i][1]):
            flag=0
if (not(N==1) and (l[0][1]==0)):
    flag=0
if flag==1:
    for i in range(N):
        if not(N==1):
            if (l[i][1]==-1):
                if i==0:
                    l[i][1]=1
                else:
                    l[i][1]=0
        else:
            if (l[i][1]==-1):
                l[i][1]=0
            l[i][1]=0
        ns="".join((ns,str(l[i][1])))
    print(ns)
else:
    print("-1")

