1 #include<algorithm>
 2 #include<iostream>
 3 #include<cstdlib>
 4 #include<cstring>
 5 #include<cstdio>
 6 #include<vector>
 7 #include<queue>
 8 #include<map>
 9 using std::cin;
10 using std::cout;
11 using std::endl;
12 using std::find;
13 using std::sort;
14 using std::map;
15 using std::pair;
16 using std::queue;
17 using std::vector;
18 using std::multimap;
19 #define pb(e) push_back(e)
20 #define sz(c) (int)(c).size()
21 #define mp(a, b) make_pair(a, b)
22 #define all(c) (c).begin(), (c).end()
23 #define iter(c) decltype((c).begin())
24 #define cls(arr,val) memset(arr,val,sizeof(arr))
25 #define cpresent(c, e) (find(all(c), (e)) != (c).end())
26 #define rep(i, n) for (int i = 0; i < (int)(n); i++)
27 #define tr(c, i) for (iter(c) i = (c).begin(); i != (c).end(); ++i)
28 const int N = 10010;
29 typedef unsigned long long ull;
30 struct Node {
31     int vex, val;
32     Node(int i = 0, int j = 0) :vex(i), val(j) {}
33 };
34 struct TopSort {
35     vector<int> G[N];
36     int topNum, inq[N];
37     inline void init(int n) {
38         topNum = 0;
39         cls(inq, 0);
40         rep(i, n) G[i].clear();
41     }
42     inline void built(int m) {
43         int a, b;
44         rep(i, m) {
45             scanf("%d %d", &a, &b);
46             --a, --b;
47             inq[a]++;
48             G[b].push_back(a);
49         }
50     }
51     inline void bfs(int n) {
52         int ans = 0;
53         queue<Node> q;
54         rep(i, n) {
55             if (!inq[i]) { q.push(Node(i, 888)); topNum++; }
56         }
57         while (!q.empty()) {
58             Node t = q.front(); q.pop();
59             ans += t.val;
60             rep(i, sz(G[t.vex])) {
61                 if (--inq[G[t.vex][i]] == 0) q.push(Node(G[t.vex][i], t.val + 1)), topNum++;
62             }
63         }
64         printf("%d\n", topNum == n ? ans : -1);
65     }
66 }work;
67 int main() {
68 #ifdef LOCAL
69     freopen("in.txt", "r", stdin);
70     freopen("out.txt", "w+", stdout);
71 #endif
72     int n, m;
73     while (~scanf("%d %d", &n, &m)) {
74         work.init(n);
75         work.built(m);
76         work.bfs(n);
77     }
78     return 0;
79 }