#include <bits/stdc++.h>
#define rank asdghasfastdd

using namespace std;

int parent[10000];
int rank[10000];

int find_set (int v) {
    if (v == parent[v])
        return v;
    return parent[v] = find_set(parent[v]);
}

void union_sets (int a, int b) {
    a = find_set(a);
    b = find_set(b);
    if (a != b) {
        if (rank[a] < rank[b])
            swap(a, b);
        parent[b] = a;
        if (rank[a] == rank[b])
            ++rank[a];
    }
}

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    int n,m,ans = -1;
    cin >> n >> m;
    for (int i = 0; i < n; ++i) {
        parent[i] = i;
    }
    for (int i = 0; i < m; ++i) {
        int x,y;
        bool b = true;
        cin >> x >> y;
        if (ans == -1) {
            union_sets(x,y);
            int t = find_set(parent[0]);
            for (int j = 1; j < n; ++j) {
                if (find_set(parent[j]) != t) b = false;
                if (!b) break;
            }
            if (b) ans = i+1;
        }
    }
    cout << ans << endl;
}