题目链接：
1 /*
2 Name:hdoj-2647-Reward
3 Copyright:
4 Author:
5 Date: 2018/4/11 15:59:18
6 Description:
7 */
8 #include <iostream>
9 #include <queue>
10 #include <vector>
11 #include <cstring>
12 #include <algorithm>
13 using namespace std;
14 const int MAXN = 1e+5;
15 int du[MAXN], n , m, L[MAXN];
16 vector<int> g[MAXN];
17 bool topsort() {
18 memset(du, 0, sizeof(du));
19 for (int i=0; i<n; i++) {
20 for (int j=0; j<g[i].size(); j++) {
21 du[g[i][j]]++;
22 }
23 }
24 int tot = 0;
25 queue<int> Q;
26 for (int i=0; i<n; i++) {
27 if (!du[i]) {
28 Q.push(i);
29 L[i] = 0;
30 }
31 }
32 while (!Q.empty()) {
33 int x = Q.front();
34 Q.pop();
35 tot++;
36 for (int j=0; j<g[x].size(); j++) {
37 int t = g[x][j];
38 du[t]--;
39 if (!du[t]) {
40 Q.push(t);
41 L[g[x][j]] = L[x] + 1;
42 }
43 }
44 }
45 if (tot == n) return 1;
46 return 0;
47 }
48 int main()
49 {
50 // freopen("in.txt", "r", stdin);
51 while (cin>>n>>m && (m || n)) {
52 memset(L, 0, sizeof(L));
53 memset(g, 0, sizeof(g));
54 for (int i=0; i<m; i++) {
55 int a, b;
56 cin>>a>>b;
57 a--;
58 b--;
59 g[b].push_back(a);
60 }
61 if (topsort() == 1) {
62 int sum = 0;
63 for (int i=0; i<n; i++) {
64 sum += L[i];
65 }
66 cout<<sum+n*888<<endl;
67 }
68 else cout<<"-1"<<endl;
69 }
70 return 0;
71 }