n, m = map(int, input().split())

s = []
for i in range(m):
    s.append(list(map(int, input().split())))

if n == 1:
    for q in range(1,10):
        o = 0
        for l, z in s:
            if q == z:
                o += 1

        if o == len(s):
            print(q)
            exit()




for j in range(int("1" + "0"*(n-1)),int("1" + "0"*n)):
    g = 0
    for k, u in s:
        if int(str(j)[k-1]) == u:
            g += 1

        if g == len(s):
            print(j)
            exit()

print(-1)