# coding: utf-8
N, M = map(int, input().split())
d = {}
ans = ["0" for _ in range(N)]
for i in range(M):
    s, c = map(int, input().split())
    if str(s) not in d.keys():
        d[str(s)] = c
        ans[s-1] = str(c)
    else:
        if d[str(s)] != c:
            print(-1)
            exit()
if "1" not in d.keys():
    ans[0] = "1"
ans = "".join(ans)
if ans[0] == "0" and N != 1:
    print(-1)
else:
    print(int(ans))