#include<iostream>

 #include<vector>

 #include<queue>

 #include<cstring>

 using namespace std;

 const int maxn=;

 int main()

 {

     int n,m;

     int sum[maxn];

     int out[maxn];

     while(cin>>n>>m)

     {

         int a,b;

         vector<int>next[maxn];

         queue<int>q;

         memset(sum,,sizeof(sum));

         memset(out,,sizeof(out));

         for(int i=;i<m;i++)

         {

             cin>>a>>b;

             next[b].push_back(a);

             out[a]++;

         }

         for(int i=;i<=n;i++)

         {

             if(out[i]==)

             {

                 q.push(i);

                 sum[i]=;

             }

         }

         if(q.empty())

         {

             cout<<-<<endl;

             continue;

         }

         int cnt=n;

         while(!q.empty())

         {

             int v=q.front();

             q.pop();

             cnt--;

             for(int i=;i<next[v].size();i++)

             {

                 out[next[v][i]]--;

                 if(out[next[v][i]]==)

                 {

                     q.push(next[v][i]);

                     sum[next[v][i]]=sum[v]+;

                 }

             }

         }

         if(cnt>)

         {

             cout<<-<<endl;

             continue;

         }

         int ans=;

         for(int i=;i<=n;i++)

         {

             ans+=sum[i];

         }

         cout<<ans<<endl;

     }

     return ;

 }