def main():
    N, M = map(int, input().split())
    P = {tuple(map(int, input().split())) for _ in range(M)}
    P = sorted(P)
    ans = [0 for _ in range(N)]

    if N > 1 and (1, 0) in P:
        print(-1)
        return

    if N > 1 and P[0][0] != 1:
        ans[0] = 1

    for i in range(len(P) - 1):
        if P[i][0] == P[i + 1][0]:
            print(-1)
            return

    for p in P:
        ans[p[0] - 1] = p[1]

    print("".join(map(str, ans)))


if __name__ == "__main__":
    main()
