1 #include<stdio.h>
 2 #include<string.h>
 3 #include<queue>
 4 using namespace std;
 5 const int MAXN=10010;
 6 struct Node{
 7     int to,next;
 8 };
 9 Node edg[MAXN*2];
10 int head[MAXN];
11 int que[MAXN];
12 queue<int>dl;
13 int money,N,M,flot;
14 void topu(){
15     int temp;
16     for(int i=1;i<=N;i++){
17         if(!que[i]){
18             dl.push(i);
19         }
20     }
21     int value=888;
22     temp=0;
23     while(!dl.empty()){
24             int t=dl.size();
25         while(t--){
26         int k=dl.front();
27         money+=value;
28         dl.pop();
29         temp++;
30         for(int j=head[k];j!=-1;j=edg[j].next){
31             que[edg[j].to]--;
32             if(!que[edg[j].to])dl.push(edg[j].to);
33         }
34         }
35         value++;
36     }
37     if(temp==N)
38     printf("%d\n",money);
39     else puts("-1");
40 }
41 void initial(){
42     memset(head,-1,sizeof(head));
43     memset(que,0,sizeof(que));
44     while(!dl.empty())dl.pop();
45     money=0;flot=1;
46 }
47 int main(){
48     int a,b;
49     while(~scanf("%d%d",&N,&M)){
50             initial();
51         for(int i=0;i<M;i++){
52             scanf("%d%d",&a,&b);
53             edg[i].to=a;
54             edg[i].next=head[b];
55             head[b]=i;
56             que[a]++;
57         }
58         topu();
59     }
60     return 0;
61 }