[n,m] = [int(i) for i in input().split()]
dic = {}
checker2 = 1
checker3 = 1
if m == 0:
    for i in range(m):
        [s,c] = [int(i) for i in input().split()]
        if s-1 in dic.keys() and dic[s-1] != str(c):
            checker2 = 0
        else:
            dic[s-1] = str(c)

    if checker2:
        for i in range(10 ** (n-1)-1,10 ** n):
            checker = 1
            num = str(i)
            for key in dic.keys():
                if key >= len(num):
                    checker = 0
                else:
                    if dic[key] != num[key]:
                        checker = 0

            if checker:
                checker3 = 0
                print(i)
                break
        if checker3:
            print(-1)

    else:
        print(-1)
else:
    print(0)