#include<iostream>#include<cstdio>#include<cstring>#include<queue>#include<algorithm>#define maxv 100500#define maxe 200500using namespace std;int n,m,nume=0,g[maxv],x[maxv],y[maxv],d[maxv],rank[maxv];int cnt[maxv];queue <int> q;struct edge{    int v,nxt;}e[maxe];void addedge(int u,int v){    e[++nume].v=v;    e[nume].nxt=g[u];    g[u]=nume;}bool check(int x){    int ret=0;while (!q.empty()) q.pop();    for (int i=1;i<=n;i++) {d[i]=0;rank[i]=-1;cnt[i]=0;}    for (int i=1;i<=x;i++) d[y[i]]++;    for (int i=1;i<=n;i++)    {        if (!d[i])        {            if (ret) return false;            q.push(i);ret++;rank[i]=1;cnt[1]++;        }    }    while (!q.empty())    {        int head=q.front();q.pop();        for (int i=g[head];i;i=e[i].nxt)        {            int v=e[i].v;            if (i>x) continue;            if (!--d[v])            {                rank[v]=rank[head]+1;cnt[rank[v]]++;                q.push(v);            }        }    }    for (int i=1;i<=n;i++)    {        if (cnt[i]!=1)            return false;    }    return true;}int main(){    scanf("%d%d",&n,&m);    for (int i=1;i<=m;i++)    {        scanf("%d%d",&x[i],&y[i]);        addedge(x[i],y[i]);    }    int l=1,r=m,ans=-1;    while (l<=r)    {        int mid=l+r>>1;        if (check(mid)) {ans=mid;r=mid-1;}        else l=mid+1;    }    printf("%d\n",ans);    return 0;}