#pragma comment(linker, "/STACK:1024000000,1024000000")

#include<cstdio>

#include<cstring>

#include<cmath>

#include<algorithm>

#include<vector>

#include<map>

#include<set>

#include<queue>

#include<stack>

#include<iostream>

using namespace std;

typedef long long LL;

const double pi=acos(-1.0),eps=1e-;

void File()

{

    freopen("D:\\in.txt","r",stdin);

    freopen("D:\\out.txt","w",stdout);

}

template <class T>

inline void read(T &x)

{

    char c=getchar(); x=;

    while(!isdigit(c)) c=getchar();

    while(isdigit(c)) {x=x*+c-''; c=getchar();}

}

const int maxn=;

int n,m,u[maxn],v[maxn];

int h[maxn],sz,r[maxn];

struct Edge{int u,v,nx;}e[maxn];

void add(int a,int b)

{

    e[sz].u=a; e[sz].v=b; e[sz].nx=h[a];

    h[a]=sz++;

}

bool check(int x)

{

    memset(h,-,sizeof h); sz=;

    memset(r,,sizeof r);

    for(int i=;i<=x;i++) add(u[i],v[i]),r[v[i]]++;

    queue<int>Q;

    int num=; for(int i=;i<=n;i++) if(r[i]==) Q.push(i),num++;

    if(num!=) return ;

    while(!Q.empty())

    {

        int top=Q.front(); Q.pop();

        num=;

        for(int i=h[top];i!=-;i=e[i].nx)

        {

            r[e[i].v]--;

            if(r[e[i].v]==)

            {

                num++;

                Q.push(e[i].v);

            }

        }

        if(num>) return ;

    }

    return ;

}

int main()

{

    scanf("%d%d",&n,&m);

    for(int i=;i<=m;i++) scanf("%d%d",&u[i],&v[i]);

    check();

    int L=,R=m,ans=-;

    while(L<=R)

    {

        int m=(L+R)/;

        if(check(m)) ans=m,R=m-;

        else L=m+;

    }

    printf("%d\n",ans);

    return ;

}