n, m = map(int, input().split())
s, c = [], []
for i in range(m):
    _s, _c = map(int, input().split())
    s.append(_s)
    c.append(_c)

for i in range(1000):
    *t, = map(int, list(str(i)))

    if len(t) != n:
        continue

    ok = True
    for j in range(m):
        if t[s[j]] != c[j]:
            ok = False
            break

    if ok:
        print(i)
        quit()

print(-1)
