#include <bits/stdc++.h>
using namespace std;
#define F first
#define S second
#define R cin>>
#define ll long long
#define ln cout<<'\n'
#define in(a) insert(a)
#define pb(a) push_back(a)
#define pd(a) printf("%.10f\n",a)
#define mem(a) memset(a,0,sizeof(a))
#define all(c) (c).begin(),(c).end()
#define iter(c) __typeof((c).begin())
#define rrep(i,n) for(ll i=(ll)(n)-1;i>=0;i--)
#define REP(i,m,n) for(ll i=(ll)(m);i<(ll)(n);i++)
#define rep(i,n) REP(i,0,n)
#define tr(it,c) for(iter(c) it=(c).begin();it!=(c).end();it++)
ll check(ll n,ll m,ll x,ll y){return x>=0&&x<n&&y>=0&&y<m;}void pr(){ln;}
template<class A,class...B>void pr(const A &a,const B&...b){cout<<a<<(sizeof...(b)?" ":"");pr(b...);}
template<class A>void PR(A a,ll n){rep(i,n)cout<<(i?" ":"")<<a[i];ln;}
const ll MAX=1e9+7,MAXL=1LL<<61,dx[8]={-1,0,1,0,-1,-1,1,1},dy[8]={0,1,0,-1,-1,1,1,-1};
typedef pair<ll,ll> P;


#define N 200001
vector<int> G[N],rG[N],g;
int n,cmp[N],u[N];

void add_edge(int x,int y){G[x].push_back(y);rG[y].push_back(x);}
void dfs(int x){u[x]=1;rep(i,G[x].size())if(!u[G[x][i]])dfs(G[x][i]);g.push_back(x);} 
void rdfs(int x,int k){u[x]=1;cmp[x]=k;rep(i,rG[x].size())if(!u[rG[x][i]])rdfs(rG[x][i],k);}
int scc() {
  fill(u,u+n,0);g.clear();rep(i,n)if(!u[i])dfs(i);fill(u,u+n,0);
  int k=0;rrep(i,g.size())if(!u[g[i]])rdfs(g[i],k++);return k;
}

void Main() {
  ll T;
  cin >> n >> T;
  vector<P> p(T);
  rep(i,T) {
    ll x,y;
    cin >> x >> y;
    x--,y--;
    p[i]=P(x,y);
  }
  ll l=0,r=T+1;
  while(l+1<r ) {
    ll m=(l+r)/2;
    rep(i,n) G[i].clear(),rG[i].clear();
    rep(i,m) add_edge(p[i].F,p[i].S);
    scc();
    set<ll> s;
    rep(i,n) s.in(cmp[i]);
    if(s.size()!=n) r=m;
    else l=m;
  }
  if(r==T+1) pr(-1);
  else pr(r);
}

int main(){ios::sync_with_stdio(0);cin.tie(0);Main();return 0;}