#include<iostream>
#include<vector>
#include<algorithm>
#include<cstring>
#include<cassert>
#include<cmath>
#include<ctime>
#include<iomanip>
#include<numeric>
#include<stack>
#include<queue>
#include<map>
#include<unordered_map>
#include<set>
#include<unordered_set>
#include<bitset>
#include<random>
using namespace std;
template<int m> struct modint
{
	private:
	unsigned int value;
	static constexpr int mod() {return m;}

	public:
	constexpr modint(const long long x = 0) noexcept
	{
		long long y = x;
		if(y < 0 || y >= mod())
		{
			y %= mod();
			if(y < 0) y += mod();
		}
		value = (unsigned int)y;
	}
	constexpr unsigned int val() noexcept {return value;}
	constexpr modint &operator+=(const modint &other) noexcept
	{
		value += other.value;
		if(value >= mod()) value -= mod();
		return *this;
	}
	constexpr modint &operator-=(const modint &other) noexcept
	{
		unsigned int x = value;
		if(x < other.value) x += mod();
		x -= other.value;
		value = x;
		return *this;
	}
	constexpr modint &operator*=(const modint &other) noexcept
	{
		unsigned long long x = value;
		x *= other.value;
		value = (unsigned int) (x % mod());
		return *this;
	}
	constexpr modint &operator/=(const modint &other) noexcept
	{
		return *this *= other.inverse();
	}
	constexpr modint inverse() const noexcept
	{
		assert(value);
		long long a = value,b = mod(),x = 1,y = 0;
		while(b)
		{
			long long q = a/b;
			a -= q*b; swap(a,b);
			x -= q*y; swap(x,y);
		}
		return modint(x);
	}
	constexpr modint power(long long N) const noexcept
	{
		assert(N >= 0);
		modint p = *this,ret = 1;
		while(N)
		{
			if(N & 1) ret *= p;
			p *= p;
			N >>= 1;
		}
		return ret;
	}
	constexpr modint operator+() {return *this;}
	constexpr modint operator-() {return modint() - *this;}
	constexpr modint &operator++(int) noexcept {return *this += 1;}
	constexpr modint &operator--(int) noexcept {return *this -= 1;}
	friend modint operator+(const modint& lhs, const modint& rhs) {return modint(lhs) += rhs;}
	friend modint operator-(const modint& lhs, const modint& rhs) {return modint(lhs) -= rhs;}
	friend modint operator*(const modint& lhs, const modint& rhs) {return modint(lhs) *= rhs;}
	friend modint operator/(const modint& lhs, const modint& rhs) {return modint(lhs) /= rhs;}
	friend ostream &operator<<(ostream &os,const modint &x) {return os << x.value;}
};
//using mint = modint<998244353>;
using mint = modint<1000000007>;
template<class S>
struct combination
{
	vector<S> f,invf;

	combination(int N = 0) : f(1,1),invf(1,1)
	{
		update(N);
	}

	void update(int N)
	{
		if((int)f.size() > N) return;
		int pi = (int)f.size();
		N = max(N,pi*2);

		f.resize(N+1),invf.resize(N+1);

		for(int i = pi;i <= N;i++) f[i] = f[i-1]*i;
		invf[N] = S(1)/f[N];
		for(int i = N-1;i >= pi;i--) invf[i] = invf[i+1]*(i+1);
	}

	S factorial(int N)
	{
		update(N);
		return f[N];
	}

	S invfactorial(int N)
	{
		update(N);
		return invf[N];
	}

	S P(int N,int K)
	{
		assert(0 <= K && K <= N);
		update(N);
		return f[N]*invf[N-K];
	}

	S C(int N,int K)
	{
		assert(0 <= K && K <= N);
		update(N);
		return f[N]*invf[K]*invf[N-K];
	}

	S H(int N,int K)
	{
		if(!N) return K == 0 ? 1:0;
		return C(N+K-1,K);
	}
};
int N,Q;
int E[1 << 17],seen[1 << 17],finished[1 << 17];
vector<int> G[1 << 17];
bool cycle(int u,int id)
{
	seen[u] = 1;
	for(int i:G[u]) if(i < id)
	{
		int v = E[i]^u;
		if(finished[v]) continue;
		if(seen[v] && !finished[v]) return true;

		if(cycle(v,id)) return true;
	}

	finished[u] = 1;
	return false;
}
void solve()
{
	cin >> N >> Q;
	for(int i = 0;i < Q;i++)
	{
		int u,v;
		cin >> u >> v;
		u--; v--;
		E[i] = u^v;
		G[u].push_back(i);
	}

	bool fn = false;
	for(int i = 0;i < N;i++) if(!seen[i] && cycle(i,Q)) fn = true;

	if(!fn)
	{
		cout << -1 << endl;
		return;
	}

	int ng = 0,ok = Q;
	while(ok-ng > 1)
	{
		int mid = (ok+ng)/2;

		bool fn = false;
		for(int i = 0;i < N;i++) seen[i] = finished[i] = 0;
		for(int i = 0;i < N;i++) if(!seen[i] && cycle(i,mid)) fn = true;

		if(fn) ok = mid;
		else ng = mid;
	}

	cout << ok << endl;
}
int main()
{
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int T = 1;
	//cin >> T;
	while(T--) solve();
}