1 #include<iostream>
 2 #include<cstdio>
 3 #include<cstring>
 4 using namespace std;
 5 #define N 10005
 6 #define M 20005
 7 int in[N];
 8 int que[N];
 9 int my[N];
10 struct Edge{
11     int to ;
12     int next;
13 }edge[M];
14 int head[N];
15 int Enct;
16 void init()
17 {
18     Enct = 0;
19     memset(head,-1,sizeof(head));
20     memset(my,0,sizeof(my));
21     memset(in,0,sizeof(in));
22 }
23 void add(int from, int to)
24 {
25     edge[Enct].to = to;
26     edge[Enct].next = head[from];
27     head[from] = Enct++;
28 }
29 int c;
30 int t;
31 int n;
32 int tm ;
33 bool tp()
34 {
35     c = 0;
36     t = 1;
37     for(int i = 0 ;i < n ;i++)
38     {
39         if(in[i]==0)
40         {
41             que[c++] = i;
42         }
43     }
44     for(int i = 0 ;i < c ; i++)
45     {
46         int id = que[i];
47         for(int j = head[id] ; j !=-1 ; j = edge[j].next)
48         {
49             int t = my[id];
50             Edge e = edge[j];
51             in[e.to]--;
52             my[e.to] = max(t+1,my[e.to]);//后继大于前驱，每次都要更新 
53             if(in[e.to]==0)
54             {
55                 que[c++] = e.to;
56             }
57         }
58     }
59     if(c<n) return false;
60     else return true;
61 } 
62 int main()
63 {
64     int m ;
65     while(~scanf("%d%d",&n,&m))
66     {
67         init();
68         for(int i = 0 ;i < m ;i++)
69         {
70             int a , b;
71             scanf("%d%d",&a,&b);
72             add(b,a);//因为是前者比后者多，所以键反向边 
73             in[b]++; 
74         }
75         if(tp()==false) printf("-1\n");
76         else 
77         {
78             int sum = 0 ;
79             for(int i = 1;i <= n ;i++)
80             {
81                 sum+=my[i];
82             }
83             printf("%d\n",888*n+sum);
84         }
85     }
86     return 0;
87 }