1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55

#include<cstdio>
#include<cstring>
#include<queue>
using namespace std;
int n;
int sum[10200];
int cnt[10200];
int head[10200];
struct node {
	int next;
	int to;
} a[20200];
int res;
int toposort() {
	res=0;
	queue<int>q;
	memset(sum,0,sizeof(sum));
	for(int i=1; i<=n; i++) {
		if(cnt[i]==0) {
			q.push(i);
			sum[i]=888;
		}
	}
	int t=0;
	while(!q.empty()) {
		int top=q.front();
		q.pop();
		t++;
		res+=sum[top];
		for(int i=head[top]; i!=-1; i=a[i].next) {
			if(--cnt[a[i].to]==0) {
				q.push(a[i].to);
				sum[a[i].to]=sum[top]+1;
			}
		}
	}
	return t<n?-1:res;
}
int main() {
	int m;
	while(scanf("%d %d",&n,&m)!=EOF) {
		memset(cnt,0,sizeof(cnt));
		memset(head,-1,sizeof(head));
		for(int i=0; i<m; i++) {
			int u,v;
			scanf("%d %d",&u,&v);
			a[i].to=u;
			a[i].next=head[v];
			head[v]=i;
			cnt[u]++;
		}
		printf("%d\n",toposort());
	}
	return 0;
}