def main():
    N, M = map(int,input().split())
    dic_number = {}
    is_impossible = False
    for _ in range(M):
        s,c = map(int,input().split())
        if s not in dic_number:
            dic_number[s] = c
        elif dic_number[s] == c:
            continue
        else:
            is_impossible = True


    if N == 1 and dic_number[1]==0:
        return 0

    if is_impossible or (dic_number[1]==0):
        return -1
    else:
        ans = 0
        for s in dic_number:
            ans += dic_number[s]*10**(N-s)

        if ans < 10**(N-1):
            ans += 10**(N-1)

        return ans

print(main())