for(int j = head[u]; edge[j].x == u && j < (int)edge.size(); j ++)
      {
          cost[edge[j].y] = max(cost[u] + 1 , cost[edge[j].y]);
          indegree[edge[j].y] --;
      }

#include <iostream>
#include <cstdio>
#include <vector>
#include <queue>
#include <algorithm>
using namespace std;
int cost[10010];
int n , m ;
typedef struct
{
   int x ;
   int y ;
}node ;
vector<node>edge;
int vis[10010];
int head[10010];
int indegree[10010];
void init()
{
    edge.clear();
    for(int i = 0; i < 10010; i ++){
        head[i] = -1;
        indegree[i] = 0;
        cost[i] = 0;
        vis[i] = 0;
    }
}

bool cmp(node a , node b)
{
    if(a.x == b.x) return a.y < b.y;
    return a.x < b.x;
}

__int64 tupo()
{
  int u ;
  for(int i = 0; i < n; i ++)
  {
      u = -1;
      int flag = 0;
      for(int j = 1; j <= n; j ++)
      {
          if(!vis[j]) flag = 1;
          if(!vis[j] && indegree[j] == 0)
          {
              u = j; break;
          }
      }
      if(flag && u == -1) return -1;
      vis[u] = 1;
      for(int j = head[u]; edge[j].x == u && j < (int)edge.size(); j ++)
      {
          cost[edge[j].y] = max(cost[u] + 1 , cost[edge[j].y]);
          indegree[edge[j].y] --;
      }
  }
  __int64 sum = 0;
  for(int i = 1; i <= n; i ++)
    sum += cost[i];
    sum += n * 888;
    return sum;
}
int main()
{
    node temp;
    while(scanf("%d%d",&n,&m)!=EOF)
    {
        init();
        for(int i = 0; i < m; i ++)
        {
            scanf("%d%d",&temp.y,&temp.x);
            edge.push_back(temp);
            indegree[temp.y] ++;
        }
        sort(edge.begin() , edge.end() , cmp);

        head[edge[0].x] = 0;
        for(int i = 1; i < (int)edge.size(); i ++)
        {
            if(edge[i].x != edge[i-1].x)
                head[edge[i].x] = i;
        }
        printf("%I64d\n",tupo());
    }
    return 0;
}