#include <iostream>
#include <fstream>
#include <cstdio>
#include <iomanip>
#include <sstream>
#include <cstring>
#include <string>
#include <cmath>
#include <stack>
#include <list>
#include <queue>
#include <deque>
#include <set>
#include <map>
#include <vector>
#include <algorithm>
#include <numeric>
#include <utility>
#include <functional>
#include <limits>
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef unsigned int ui;
typedef pair<int,int> pii;
typedef vector<vector<int> > graph;

const double pi = acos(-1.0);

#define oned(a, x1, x2) { cout << #a << ":"; for(int _i = (x1); _i < (x2); _i++){ cout << " " << a[_i]; } cout << endl; }
#define twod(a, x1, x2, y1, y2) { cout << #a << ":" << endl; for(int _i = (x1); _i < (x2); _i++){ for(int _j = (y1); _j < (y2); _j++){ cout << (_j > y1 ? " " : "") << a[_i][_j]; } cout << endl; } }

#define mp make_pair
#define pb push_back
#define fst first
#define snd second

#define MAXN 300005

int parent[MAXN];

int Group(int a) {
	if(parent[a]==a) {
		return a;
	} else {
		return parent[a] = Group(parent[a]);
	}
}

int n, m, x[MAXN], y[MAXN];

int adj[MAXN];

void solve() {
	for(int i = 1; i <= n; i++) {
		parent[i] = i;
	}
	
	memset(adj,0,sizeof(adj));
	for(int i = 1; i <= m; i++) {
		int a = Group(x[i]);
		int b = Group(y[i]);
		if(a==b) {
			cout << i << endl;
			return;
		} else {
			if(adj[a]) {
				parent[adj[a]] = b;
			}
			if(adj[b]) {
				parent[adj[b]] = a;
			}
			adj[a] = b;
			adj[b] = a;
		}
	}
}

int main() {
	ios_base::sync_with_stdio(false);
//	freopen("input.in", "r", stdin);
	while(cin >> n >> m) {
		for(int i = 1; i <= m; i++) {
			cin >> x[i] >> y[i];
		}
		solve();
	}
}