#include <iostream>
#include <cstdio>
#include <string.h>
#include <queue>
#include <algorithm>
#include <math.h>
using namespace std;
const int N = 10005;
const int M = 20005;
struct Edge{
    int v,next;
}edge[M];
int head[N];
int indegree[N];
int w[N];
int tot;
void init(){
    memset(head,-1,sizeof(head));
    memset(indegree,0,sizeof(indegree));
    tot = 0;
}
void addEdge(int u,int v,int& tot){
    edge[tot].v = v,edge[tot].next = head[u],head[u] = tot++;
}
int main(){
    int n,m;
    while(scanf("%d%d",&n,&m)!=EOF){
        init();
        for(int i=1;i<=m;i++){
            int u,v;
            scanf("%d%d",&u,&v);
            addEdge(v,u,tot);
            indegree[u]++;
        }
        queue<int> q;
        int tot = 0;
        for(int i=1;i<=n;i++){
            if(!indegree[i]) {
                q.push(i);
                w[i] = 888;
                tot+=w[i];
            }
        }
        int cnt = 0;
        while(!q.empty()){
            int u = q.front();
            q.pop();
            cnt++;
            for(int k = head[u];k!=-1;k=edge[k].next){
                int v = edge[k].v;
                indegree[v]--;
                if(!indegree[v]){
                    w[v] = w[u]+1;
                    tot+=w[v];
                    q.push(v);
                }
            }
        }
        if(cnt!=n) printf("-1\n");
        else{
            printf("%d\n",tot);
        }
    }
    return 0;
}