def main():
    r1 = input()
    n, m = map(int, r1.split(" "))
    v = ["~"] * n
    for t in range(m):
        r = input()
        s, c = map(int, r.split(" "))
        s -= 1
        if v[s] != "~" and v[s] != str(c):
            print(-1)
            return
        v[s] = str(c)
    for x in range(len(v)):
        if v[x] == '~':
            v[x] = "0"
    sv = "".join(v)
    res = int(sv)
    res_str = str(res)
    if len(res_str) != n:
        print(-1)
        return
    print(res_str)

main()
    