N,M = map(int,input().split())
s = []
c = []
ans = -1
ans_flg = True
for i in range(M):
  si, ci = map(int,input().split())
  s.append(si)
  c.append(ci)
for i in range(1000):
    #print("i : ",i)
    ans_str = str(i)
    if i < 10**(N-1) or i > 10**(N+1):
        continue
    for j in range(len(s)):
        #print("ans_str : ",ans_str,", ans_str[s[j]-1] : ",ans_str[s[j]-1],", s[j] : ",s[j],", c[j] : ",c[j])
        if ans_str[s[j]-1] != str(c[j]):
            #print("hoge",ans_flg)
            ans_flg = False
            break
        else:
            #print("fuga")
            ans_flg = True
    #print(i)
    if ans_flg:
        ans = i
        break
print(ans)
