N, M = map(int, input().split())
s = [list(map(int, list(input().split()))) for i in range(M)]

ans = [-1] * M
for i in s:
    if ans[i[0] - 1] != -1 and ans[i[0] - 1] != i[1]:
        print(-1)
        exit()
    ans[i[0] - 1] = i[1]

ans = [int(str(a).replace('-1', '0')) for a in ans]
res = ''.join((map(str, ans)))
if int(res) != 0:
    print(int(res))
else:
    print(-1)
