
    N, M = map(int, input().split())
    s_list = list()
    c_list = list()
    for _ in range(M):
        s, c = input().split()
        s_list.append(s)
        c_list.append(c)

    out = '-1'
    exists = True

    num = 'd' * N

    for index in range(len(s_list)):
        if num[int(s_list[index]) - 1] == 'd' or num[int(s_list[index]) - 1] == c_list[index]:
            # num[int(s_list[index])] = c_list[index]
            num_tmp = num[:int(s_list[index]) - 1]
            num_tmp += c_list[index] + num[int(s_list[index]):]
            num = num_tmp

        else:
            exists = False
            break

    if exists:
        out = num.replace('d', '0')
        if out[0] == '0' and N > 1:
            out = '-1'

    print(out)