1 #include <queue>
 2 #include <cstdio>
 3 #include <cstring>
 4 #include <iostream>
 5 using namespace std;
 6 int indegree[10010], mon[10010]; 
 7 int n, m; 
 8 struct Edge
 9 {
10     int from, to, next;
11 } edge[20020];
12 int head[10010], cnt; 
13 void Add(int a, int b)
14 {
15     Edge E = {a, b, head[a]}; 
16     edge[cnt] = E;
17     head[a] = cnt++;
18 } 
19 int sum;
20 void Tsort()
21 {
22     sum = 0;
23     queue<int> q;
24     for(int i = 1; i <= n; i++){
25         if(!indegree[i])
26             q.push(i);
27     mon[i] = 888;
28     }
29     while(!q.empty())
30     {
31         int u = q.front();
32         q.pop(); indegree[u]--;  sum++;
33         for(int i = head[u]; i != -1; i = edge[i].next)
34         {
35             if(!--indegree[edge[i].to])
36                 mon[edge[i].to] = mon[u] + 1, q.push(edge[i].to); 
37         }
38     }
39     int total = 0;
40     if(sum == n)
41     {
42         for(int i = 1; i <= n; i++)
43             total += mon[i];
44         printf("%d\n", total);
45     }
46     else
47         printf("-1\n"); 
48 }
49 int main()
50 {
51     while(~scanf("%d %d", &n, &m))
52     {
53         cnt = 0;
54         memset(head, -1, sizeof(head));
55         memset(indegree, 0, sizeof(indegree));
56         for(int i = 1; i <= m; i++)
57         {
58             int a, b;
59             scanf("%d %d", &a, &b);
60             indegree[a]++;
61             Add(b, a);    
62         } 
63         Tsort();
64     } 
65     return 0;    
66 }