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

fail = False
result = {}
for i in range(0, m):
    s, c = map(int, input().split())
    key = n-s
    if key in result and result[key] != c:
        fail = True
        break
    result[key] = c

if not(n-1 in result) or result[n-1] == 0:
   fail = True

if fail:
    print(-1)
else:
    res = 0
    for i in range(0, n):
        if i in result:
            res += 10 ** i * result[i]
    print(res)
