#define _CRT_SECURE_NO_WARNINGS
#define debug(tl) cerr<<#tl<<' '<<tl<<'\n';
#define de(tl) cerr<<#tl<<' '<<tl<<"   ";
#include "bits/stdc++.h"
using namespace std;
#define all(mask) mask.begin(), mask.end()
typedef long long ll;
typedef pair<ll, ll> pll;
typedef pair<int, int> pii;
typedef long double ld;
struct dsu {
    vector<int> p, siz;
    dsu(int n) {
        p.resize(n), siz.resize(n);
        for (int i = 0; i < n; i++) {
            p[i] = i;
            siz[i] = 1;
        }
    }
    int find(int v) {
        if (v == p[v]) {
            return v;
        }
        return p[v] = find(p[v]);
    }
    void union_set(int a, int b) {
        a = find(a);
        b = find(b);
        if (a == b) {
            return;
        }
        else {
            if (siz[a] < siz[b]) {
                swap(a, b);
            }
            p[b] = a;
            siz[a] += siz[b];
        }
    }
};
signed main() {
#ifdef _DEBUG
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
#endif
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    int n, m;
    cin >> n >> m;
    dsu U(n);
    int cnt = 0, ans = 0;
    bool fl = false;
    if (n == 1)fl = true;
    for (int i = 0; i < m; i++) {
        int a, b;
        cin >> a >> b;
        a--, b--;
        if (U.find(a) != U.find(b)) {
            cnt++;
            U.union_set(a, b);
        }
        if (!fl && cnt == n - 1) {
            ans = i + 1;
            fl = true;
        }
    }
    cout << ans << '\n';
}