n,m = map(int,input().split())
ans = [0]*n
for _ in range(m):
    s,c = map(int,input().split())
    if ans[s-1] == 0:
        ans[s-1] = c
    elif ans[s-1] != 0 and ans[s-1] == c:
        pass
    elif ans[s-1] != 0 and ans[s-1] != c:
        print(-1)
        exit()
if ans[0] == 0:
    print(-1)
    exit()
else:
    ans = map(str,ans)
    ans = "".join(ans)
    print(ans)