n, m = map(int, input().split())
sc = [list(map(int, input().split())) for i in range(m)]
l = []
l2 = []
l3 = [0 for i in range(n)]
cnt = 0
if m == 0 or (n == 1 and m == 1 and sc[0] == [1, 0]):
    print(0)
    exit()
for i in sc:
    if i[0] == 1 and i[1] == 0:
        print(-1)
        exit()
    elif i[0] in l and l2[l.index(i[0])] != i[1]:
        print(-1)
        exit()
    else:
        l.append(i[0])
        l2.append(i[1])
        l3[i[0] - 1] = i[1]
        cnt += 1
if l3[0] == 0:
    l3[0] = 1
print(*l3, sep="")