#include <iostream>
#include <unordered_map>
#include <unordered_set>
#include <set>
#include <vector>
#include <numeric>
#include <algorithm>
#include <queue>
#include <string>
#include <random>
#include <array>
#include <climits>
#include <map>
#include <cassert>
#include <stack>
#include <iomanip>
#include <cfloat>
#include <bitset>
#include <fstream>
#include <chrono>


bool is_dag(const std::vector<std::vector<int>>& graph) {
	const int n = graph.size();
	std::vector<bool> visit(n, false), out(n, false);
	std::stack<int> stack;
	for (auto i = 0; i < n; ++i) {
		if (visit[i]) continue;
		stack.push(i);
		while (!stack.empty()) {
			const auto current = stack.top(); stack.pop();
			if (current >= 0) {
				if (visit[current]) {
					if (!out[current]) return false;
					continue;
				}
				visit[current] = true;
				stack.push(-1 - current);
				for (const auto next : graph[current]) {
					stack.push(next);
				}
			}
			else {
				out[-1 - current] = true;
			}
		}
	}
	return true;
}
int main() {
	int n, q; std::cin >> n >> q;
	std::vector<std::pair<int, int>> edges(q);
	for (auto& [a, b] : edges) {
		std::cin >> a >> b; --a; --b;
	}
	int min = 1;
	int max = q + 1;
	while (min < max) {
		auto mid = (min + max) >> 1;
		std::vector<std::vector<int>> graph(n);
		for (int e = 0; e < mid; ++e) {
			const auto [a, b] = edges[e];
			graph[a].push_back(b);
		}
		if (is_dag(graph)) {
			min = mid + 1;
		}
		else {
			max = mid;
		}
	}
	if (max <= q) {
		std::cout << max << '\n';
	}
	else {
		std::cout << "-1\n";
	}
}