#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;

//template
#define rep(i,a,b) for(int i=(int)(a);i<(int)(b);i++)
#define ALL(v) (v).begin(),(v).end()
typedef long long int ll;
const int inf = 0x3fffffff; const ll INF = 0x1fffffffffffffff; const double eps=1e-12;
template<typename T>inline bool chmax(T& a,T b){if(a<b){a=b;return 1;}return 0;}
template<typename T>inline bool chmin(T& a,T b){if(a>b){a=b;return 1;}return 0;}
//end

typedef pair<int,int> P;
int n,m; vector<P> g[501010];
vector<int> res;
int used[501010]; P par[501010];
bool dfs(int v){
   used[v]=1;
   for(auto& e:g[v]){
      par[v]={e.first,e.second};
      if(used[e.first]==0 and dfs(e.first))return 1;
      else if(used[e.first]==1){
         int cur=e.first;
         do{
            res.push_back(par[cur].second);
            cur=par[cur].first;
         }while(cur!=e.first);
         return 1;
      }
   } used[v]=2; return 0;
}
bool cyclic(){
   rep(i,0,n)used[i]=0,par[i]={-1,-1};
   rep(i,0,n)if(used[i]==0 and dfs(i))return 1;
   return 0;
}

int main(){
   scanf("%d%d",&n,&m);
   vector<int> h(n);
   rep(i,0,m){
      int x,y; scanf("%d%d",&x,&y);
      x--; y--;
      g[x].push_back({y,i});
      h[y]++;
   }
   if(cyclic()){
      puts("-1");
   }
   else{
      queue<int> none;
      rep(i,0, n) {
         if (h[i] == 0) {
            none.push(i);
         }
      }
      vector<int> tsort;
      while (none.size()) {
         int i = none.front(); none.pop();
         tsort.push_back(i);
         for (auto& [j,tmp] : g[i]) {
            h[j]--;
            if (h[j] == 0) none.push(j);
         }
      }
      reverse(tsort.begin(), tsort.end());
      vector<int> dp(n, 0);
      int ans = 0;
      for (int i : tsort) {
         for(auto& [u,tmp]:g[i]){
            dp[i] = max(dp[i], dp[u] + 1);
         }
         ans = max(ans, dp[i]);
      }
      cout << ans+1 << endl;
      return 0;
   }
   return 0;
}