#include<stdio.h>
#include<queue>
#include<iostream>
using namespace std;
#define N 11000
int n,m,sum;
struct node {
int indegree,num;
int value,next[300];
}now[N];
int Max(int a,int b) {
return a>b?a:b;
}
int find() {
queue<node>q;
node cur;
int i,f=0;
for(i=1;i<=n;i++)
if(now[i].indegree==0) {
now[i].value=888;
f=1;
q.push(now[i]);
}
if(f==0)
return 1;
f=n;
while(!q.empty()) {
cur=q.front();
q.pop();
f--;
for(i=0;i<cur.num;i++) {
now[cur.next[i]].value=Max(now[cur.next[i]].value,cur.value+1);
if(--now[cur.next[i]].indegree==0)
q.push(now[cur.next[i]]);
}
}
if(f==0)
return 0;
return 1;
}
int main() {
int i,a,b;
while(scanf("%d%d",&n,&m)!=EOF) {
for(i=1;i<=n;i++) {
now[i].num=0;
now[i].indegree=0;
now[i].value=0;
memset(now[i].next,0,sizeof(now[i].next));
}
while(m--) {
scanf("%d%d",&a,&b);
now[a].indegree++;
now[b].next[now[b].num++]=a;
}
if(find()) {
printf("-1
");
continue;
}
sum=0;
for(i=1;i<=n;i++)
sum+=now[i].value;
printf("%d
",sum);
}
return 0;
}