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

for i in range(1000):
    t = str(i)
    if all((len(t) > s-1) and int(t[s-1]) == (c)  for s, c in SC):
        print(i)
        exit()
print(-1)
